Display device and method for manufacturing the same

ABSTRACT

With an increase in the definition of a display device, the number of pixels is increased, and thus the numbers of gate lines and signal lines are increased. The increase in the numbers of gate lines and signal lines makes it difficult to mount an IC chip having a driver circuit for driving the gate line and the signal line by bonding or the like, which causes an increase in manufacturing costs. A pixel portion and a driver circuit driving the pixel portion are provided over the same substrate. The pixel portion and at least a part of the driver circuit are formed using thin film transistors in each of which an oxide semiconductor is used. Both the pixel portion and the driver circuit are provided over the same substrate, whereby manufacturing costs are reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device in which an oxidesemiconductor is used and a method for manufacturing the same.

2. Description of the Related Art

A thin film transistor formed over a flat plate such as a glasssubstrate is manufactured using amorphous silicon or polycrystallinesilicon, as typically seen in a liquid crystal display device. A thinfilm transistor manufactured using amorphous silicon has low fieldeffect mobility, but such a transistor can be formed over a glasssubstrate with a larger area. On the other hand, a thin film transistormanufactured using a crystalline silicon has high field effect mobility,but due to a crystallization step such as laser annealing, such atransistor is not always suitable for being formed over a larger glasssubstrate.

In view of the foregoing, attention has been drawn to a technique bywhich a thin film transistor is manufactured using an oxidesemiconductor and such a transistor is applied to an electronic deviceor an optical device. For example, Patent Document 1 and Patent Document2 disclose a technique by which a thin film transistor is manufacturedusing zinc oxide (ZnO) or an In—Ga—Zn—O based oxide semiconductor as anoxide semiconductor film and such a transistor is used as a switchingelement or the like of an image display device.

CITATION LIST Patent Document 1

-   Japanese Published Patent Application No. 2007-123861

Patent Document 2

-   Japanese Published Patent Application No. 2007-096055

SUMMARY OF THE INVENTION

The field effect mobility of a thin film transistor using an oxidesemiconductor for a channel formation region is higher than that of athin film transistor using amorphous silicon. The oxide semiconductorfilm can be formed by sputtering or the like at a temperature of lowerthan or equal to 300° C. Its manufacturing process is easier than thatof a thin film transistor using polycrystalline silicon.

Such an oxide semiconductor is expected to be used for forming a thinfilm transistor on a glass substrate, a plastic substrate, or the like,and to be applied to a liquid crystal display device, anelectroluminescent display device, an electronic paper, or the like.

With an increase in the definition of a display device, the number ofpixels is increased, and thus the numbers of gate lines and signal linesare increased. The increase in the numbers of gate lines and signallines makes it difficult to mount an IC chip having a driver circuit fordriving the gate lines and the signal lines by bonding or the like,which causes an increase in manufacturing costs.

Thus, it is an object of the present invention to reduce manufacturingcosts by use of an oxide semiconductor for at least a part of a drivercircuit for driving a pixel portion.

Further, it is another object of the present invention to reduce contactresistance or the like between wirings that connect elements in order toachieve high-speed driving of the driver circuit. For example, highcontact resistance between a gate wiring and an upper wiring mightdistort an inputted signal.

Further, it is another object of the present invention to provide astructure of a display device, which is capable of reducing the numberof contact holes and an area occupied by driver circuits.

In an embodiment of the present invention, a pixel portion and at leasta part of a driver circuit for driving the pixel portion are formedusing thin film transistors in each of which an oxide semiconductor isused over the same substrate. Both the pixel portion and the drivercircuit are provided over the same substrate, whereby manufacturingcosts are reduced.

As an oxide semiconductor used in this specification, a thin film of amaterial represented by InMO₃(ZnO)_(m) (m>0) is formed, and a thin filmtransistor in which the thin film is used as a semiconductor layer ismanufactured. Note that M denotes one or more of metal elements selectedfrom gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), and cobalt(Co). In addition to a case where only Ga is contained as M, there is acase where Ga and the above metal elements other than Ga, for example,Ga and Ni or Ga and Fe are contained as M. Moreover, in the oxidesemiconductor, in some cases, a transition metal element such as Fe orNi or an oxide of the transition metal is contained as an impurityelement in addition to a metal element contained as M. In thisspecification, this thin film is also referred to as an “In—Ga—Zn—Onon-single-crystal film”.

Table 1 shows a typical example of measurement by inductively coupledplasma mass spectrometry (ICP-MS). An oxide semiconductor film that isobtained in Condition 1 where a target (In:Ga:Zn=1:1:0.5) in whichIn₂O₃, Ga₂O₃, and NzO are contained at a ratio of 1:1:1 and a flow rateof an argon gas in a sputtering method is 40 sccm isInGa_(0.95)Zn_(0.41)O_(3.33). In addition, an oxide semiconductor filmobtained in Condition 2 where flow rates of an argon gas and oxygen in asputtering method are 10 sccm and 5 sccm respectively isInGa_(0.94)Zn_(0.40)O_(3.31).

TABLE 1 Flow ratio Composition (atomic %) Ar/O₂ In Ga Zn O Compositionformula 40/0 17.6 16.7 7.2 58.6 InGa_(0.95)Zn_(0.41)O_(3.33) 10/5 17.716.7 7 58.6 InGa_(0.94)Zn_(0.40)O_(3.31)

Further, Table 2 shows results of quantification performed usingRutherford backscattering spectrometry (RBS) instead of ICP-MS.

TABLE 2 Flow ratio Composition (atomic %) Ar/O₂ In Ga Zn O ArComposition formula 40/0 17 15.8 7.5 59.4 0.3InGa_(0.93)Zn_(0.44)O_(3.49) 10/5 16 14.7 7.2 61.7 0.4InGa_(0.92)Zn_(0.45)O_(3.86)

According to the results of the measurement of the sample in Condition 1by RBS, an oxide semiconductor film is InGa_(0.93)Zn_(0.44)O_(3.49). Inaddition, according to the results of the measurement of the sample inCondition 2 by RBS, an oxide semiconductor film isInGa_(0.92)Zn_(0.45)O_(3.86).

An amorphous structure is observed in the In—Ga—Zn—O non-single-crystalfilm by X-ray diffraction (XRD). Note that heat treatment is performedon the In—Ga—Zn—O non-single-crystal film of the examined sample at 200°C. to 500° C., typically 300° C. to 400° C., for 10 minutes to 100minutes after the film is formed by a sputtering method. In addition, athin film transistor having electric characteristics such as an on/offratio of greater than or equal to 10⁹ and a mobility of greater than orequal to 10 at a gate voltage of ±20 V can be manufactured.

It is useful to use the thin film transistor having such electriccharacteristics for a driver circuit. For example, a gate line drivercircuit includes a shift register circuit for sequentially transferringa gate signal, a buffer circuit, and the like; and a source line drivercircuit includes a shift register circuit for sequentially transferringa gate signal, an analog switch for switching on and off of transfer ofan image signal to a pixel, and the like. A TFT in which an oxidesemiconductor film having a higher mobility than a TFT in whichamorphous silicon is used is capable of driving a shift register circuitat high speed.

Further, in a case where at least a part of a driver circuit for drivinga pixel portion is formed using a thin film transistor in which an oxidesemiconductor is used, the circuit is formed using n-channel TFTs, and acircuit illustrated in FIG. 1B is used as a basic unit. In addition, inthe driver circuit, a gate electrode is directly connected to a sourcewiring or a drain wiring, whereby a favorable contact can be obtained,which leads to reduction in contact resistance. In a case where, in thedriver circuit, the gate electrode is connected to a source wiring or adrain wiring through another conductive film, for example, a transparentconductive film, an increase in the number of contact holes, an increasein an area occupied by the contact holes due to the increase in thenumber of contact holes, or an increase in contact resistance and wiringresistance might be caused, and furthermore, an increase in complexitymight be caused.

According to a structure of the present invention disclosed in thisspecification, a display device includes a pixel portion and a drivercircuit, where the pixel portion includes a first thin film transistorhaving at least a first oxide semiconductor layer, the driver circuitincludes a second thin film transistor having at least a second oxidesemiconductor layer and a third thin film transistor having a thirdoxide semiconductor layer, a wiring which is directly connected to agate electrode of the second thin film transistor provided below thesecond oxide semiconductor layer is a source wiring or a drain wiring ofthe third thin film transistor which is electrically connected to thethird oxide semiconductor layer, and the third oxide semiconductor layeris on and in direct contact with the wiring.

An embodiment of the present invention achieves at least one of theabove objects.

Further, in the above structure, the gate electrode of the second thinfilm transistor is electrically connected to the wiring through acontact hole formed in a gate insulating layer that covers the gateelectrode. In addition, in the above structure, the pixel portion andthe driver circuit are formed over the same substrate, wherebymanufacturing costs are reduced.

Further, since the thin film transistor is easily broken by staticelectricity and the like, a protection circuit for protecting the drivercircuits is preferably provided over the same substrate for a gate lineor a source line. The protection circuit is preferably formed using anonlinear element in which an oxide semiconductor is used.

Moreover, as a display device including a driver circuit, alight-emitting display device in which a light-emitting element is usedand a display device in which an electrophoretic display element isused, which is also referred to as an “electronic paper”, are given inaddition to a liquid crystal display device.

In the light-emitting display device in which a light-emitting elementis used, a plurality of thin film transistors are included in a pixelportion, and also in the pixel portion, there is a region where a gateelectrode of one thin film transistor is directly connected to a sourcewiring or a drain wiring of another transistor. In addition, in thedriver circuit of the light-emitting display device in which alight-emitting element is used, there is a region where a gate electrodeof a thin film transistor is directly connected to a source wiring or adrain wiring of the thin film transistor.

Further, a manufacturing method is also an embodiment of the presentinvention. The manufacturing method includes the steps of: forming afirst gate electrode and a second gate electrode over a substrate;forming a gate insulating layer that covers the first gate electrode andthe second gate electrode; forming a contact hole that reaches thesecond gate electrode by selective etching of the gate insulating layer;forming a first wiring that is in direct contact with the second gateelectrode through the contact hole and a second wiring that overlapswith both the first gate electrode and the second gate electrode throughthe first gate insulating layer; and forming a first oxide semiconductorlayer that overlaps with the first gate electrode over the gateinsulating layer and a second oxide semiconductor layer that overlapswith the second gate electrode over the gate insulating layer. Thesecond oxide semiconductor layer is on and in direct contact with thefirst wiring and the second wiring. The above structure of themanufacturing method enables an inverter circuit which is a basic unitof a driver circuit to be manufactured.

Needless to say, the thin film transistor in the pixel portion as wellas the driver circuit can also be formed over the same substrate.

Further, in the above manufacturing process, plasma treatment,specifically, reverse sputtering is preferably performed on a surface ofthe gate insulating layer to remove dust or the like on the surfacebefore the first oxide semiconductor layer and the second oxidesemiconductor layer are formed. In addition, plasma treatment,specifically, reverse sputtering is preferably performed on the surfaceof the gate insulating layer and a surface of the second gate electrodewhich is exposed at a bottom surface of the contact hole to remove dustor the like on the surfaces before the first wiring and the secondwiring are formed.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

A thin film transistor in which an oxide semiconductor is used in a gateline driver circuit or a source line driver circuit, wherebymanufacturing costs are reduced. Moreover, a gate electrode of the thinfilm transistor used for the driver circuit is directly connected to asource wiring or a drain wiring, whereby a display device in which thenumber of contact holes can be reduced and an area occupied by thedriver circuit is reduced can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A is a cross-sectional view of a semiconductor device of anembodiment of the present invention, FIG. 1B is an equivalent circuitdiagram, and FIG. 1C is a top view of the same;

FIG. 2A is an equivalent circuit diagram and FIG. 2B is a top viewthereof;

FIGS. 3A to 3C are cross-sectional views illustrating a manufacturingprocess of a semiconductor device of an embodiment of the presentinvention;

FIGS. 4A to 4C are cross-sectional views illustrating a manufacturingprocess of a semiconductor device of an embodiment of the presentinvention;

FIGS. 5A to 5C are cross-sectional views illustrating a manufacturingprocess of a semiconductor device of an embodiment of the presentinvention;

FIG. 6 is a top view of a semiconductor device of an embodiment of thepresent invention;

FIG. 7 is a top view of a semiconductor device of an embodiment of thepresent invention;

FIG. 8 is a top view of a semiconductor device of an embodiment of thepresent invention;

FIG. 9 is a top view of a semiconductor device of an embodiment of thepresent invention;

FIGS. 10A and 10B are each a cross-sectional view of a semiconductordevice of an embodiment of the present invention and FIGS. 10C and 10Dare top views of the same;

FIG. 11 is a top view of a pixel of a semiconductor device of anembodiment of the present invention;

FIG. 12 is a cross-sectional view of an electronic paper;

FIGS. 13A and 13B are block diagrams of a semiconductor device;

FIG. 14 is a diagram illustrating a source line driver circuit;

FIG. 15 is a timing chart illustrating operation of a source line drivercircuit;

FIG. 16 is a timing chart illustrating operation of a source line drivercircuit;

FIG. 17 is a diagram illustrating a structure of a shift register;

FIG. 18 is a diagram illustrating a connection structure of a flip flopillustrated in FIG. 17;

FIGS. 19A and 19B are each a top view illustrating a semiconductordevice of an embodiment of the present invention and FIG. 19C is across-sectional view illustrating the same;

FIG. 20 is a cross-sectional view illustrating a semiconductor device ofan embodiment of the present invention;

FIG. 21 is a diagram illustrating a pixel equivalent circuit of asemiconductor device of an embodiment of the present invention;

FIGS. 22A to 22C are each a view illustrating a semiconductor device ofan embodiment of the present invention;

FIG. 23A is a top view illustrating a semiconductor device of anembodiment of the present invention and FIG. 23B is a cross-sectionalview illustrating the same;

FIGS. 24A and 24B are views illustrating examples of usage patterns ofan electronic paper;

FIG. 25 is an external view illustrating an example of an e-book reader;

FIGS. 26A and 26B are external views illustrating a television set and adigital photo frame, respectively;

FIGS. 27A and 27B are external views illustrating examples of gamemachines;

FIGS. 28A and 28B are external views illustrating examples of mobilephones;

FIG. 29 is a graph showing V_(G)-I_(D) curve that is TFT electricalcharacteristics;

FIG. 30 is a graph showing results of measurement with an oscilloscope,which shows output waveforms of Units 42 to 44 of a shift register;

FIG. 31 is a graph showing results of measurement with an oscilloscope,which shows output waveforms at the time of the maximum drivingfrequency; and

FIG. 32 is a view showing how a liquid crystal display displays animage.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be hereinafter described.

Embodiment 1

In Embodiment 1, an embodiment of the present invention will bedescribed based on an example in which an inverter circuit is formedusing two n-channel thin film transistors.

A driver circuit for driving a pixel portion is formed using an invertercircuit, a capacitor, a resistor, and the like. In a case where twon-channel TFTs are combined to form an inverter circuit, there are twotypes of combinations: a combination of an enhancement type transistorand a depression type transistor (hereinafter, a circuit formed by sucha combination is referred to as an “EDMOS circuit”) and a combination ofenhancement type TFTs (hereinafter, a circuit formed by such acombination is referred to as an “EEMOS circuit”). Note that in a casewhere the threshold voltage of the n-channel TFT is positive, then-channel TFT is defined as an enhancement type transistor, while in acase where the threshold voltage of the n-channel TFT is negative, then-channel TFT is defined as a depression type transistor, and thisspecification follows the above definitions.

The pixel portion and the driver circuit are formed over the samesubstrate. In the pixel portion, on and off of voltage application to apixel electrode are switched using enhancement type transistors arrangedin a matrix. An oxide semiconductor is used for these enhancement typetransistors arranged in the pixel portion. Since the enhancement typetransistor has electric characteristics such as an on/off ratio ofgreater than or equal to 10⁹ at a gate voltage of ±20 V, leakage currentis small and low power consumption drive can be realized.

FIG. 1A illustrates a cross-sectional structure of the inverter circuitof the driver circuit. In FIG. 1A, a first gate electrode 401 and asecond gate electrode 402 are provided over a substrate 400. The firstgate electrode 401 and the second gate electrode 402 can be formed tohave a single-layer structure or a stacked-layer structure using a metalmaterial such as molybdenum, titanium, chromium, tantalum, tungsten,aluminum, copper, neodymium, or scandium, or an alloy materialcontaining any of these materials as its main component.

For example, as a two-layer structure of each of the first gateelectrode 401 and the second gate electrode 402, the following two-layerstructures are preferable: a two-layer structure of an aluminum layerand a molybdenum layer thereover, a two-layer structure of a copperlayer and a molybdenum layer thereover, a two-layer structure of acopper layer and a titanium nitride layer or a tantalum nitride layerthereover, and a two-layer structure of a titanium nitride layer and amolybdenum layer. As a three-layer structure, the following structure ispreferable: a stacked layer of a tungsten layer or a tungsten nitridelayer, an alloy layer of aluminum and silicon or an alloy layer ofaluminum and titanium, and a titanium nitride layer or a titanium layer

Further, a first wiring 409, a second wiring 410, and a third wiring 411are provided over a gate insulating layer 403 that covers the first gateelectrode 401 and the second gate electrode 402. The second wiring 410is directly connected to the second gate electrode 402 through a contacthole 404 formed in the gate insulating layer 403.

Further, a first oxide semiconductor layer 405 which is on the firstwiring 409 and the second wiring 410 is provided at a positionoverlapping with the first gate electrode 401, and a second oxidesemiconductor layer 407 which is on the second wiring 410 and the thirdwiring 411 is provided at a position overlapping with the second gateelectrode 402. Note that plasma treatment is preferably performed on asurface of the gate insulating layer 403 before the first oxidesemiconductor layer 405 and the second oxide semiconductor layer 407 areformed. For example, reverse sputtering in which plasma is generated byintroduction of an argon gas is preferably performed to remove dustattached to the surface of the gate insulating layer 403 and a bottomsurface of the contact hole 404 before the oxide semiconductor film isformed by a sputtering method. The reverse sputtering is a method inwhich voltage is applied to a substrate side, not to a target side, inan argon atmosphere and plasma is generated so that a substrate surfaceis modified. Note that nitrogen, helium, or the like may be used insteadof an argon atmosphere. Alternatively, the reverse sputtering may beperformed in an argon atmosphere to which oxygen, hydrogen, N₂O, or thelike is added. Further alternatively, the reverse sputtering may beperformed in an argon atmosphere to which Cl₂, CF₄, or the like isadded.

A first thin film transistor 430 includes the first gate electrode 401and the first oxide semiconductor layer 405 that overlaps with the firstgate electrode 401 with the gate insulating layer 403 interposedtherebetween, and the first wiring 409 is a power supply line at aground potential (a ground power supply line). This ground potentialpower supply line may be a power supply line to which negative voltageVDL is applied (a negative power supply line).

Further, a second thin film transistor 431 includes the second gateelectrode 402 and the second oxide semiconductor layer 407 that overlapswith the second gate electrode 402 with the gate insulating layer 403interposed therebetween, and the third wiring 411 is a power supply lineto which positive voltage VDD is applied (a positive power supply line).

As illustrated in FIG. 1A, the second wiring 410 which is electricallyconnected to both the first oxide semiconductor layer 405 and the secondoxide semiconductor layer 407 is directly connected to the second gateelectrode 402 of the second thin film transistor 431 through the contacthole 404 formed in the gate insulating layer 403. The second wiring 410and the second gate electrode 402 are directly connected to each other,whereby favorable contact can be obtained, which leads to a reduction incontact resistance. In comparison with a case where the second gateelectrode 402 and the second wiring 410 are connected to each other withanother conductive film, for example, a transparent conductive filminterposed therebetween, a reduction in the number of contact holes anda reduction in an area occupied by the driver circuit by the reductionin the number of contact holes can be achieved.

Further, FIG. 1C is a top view of the inverter circuit of the drivercircuit. In FIG. 1C, a cross section taken along the chain line Z1-Z2corresponds to FIG. 1A.

Further, FIG. 1B illustrates an equivalent circuit of the EDMOS circuit.The circuit connection illustrated in FIGS. 1A and 1C corresponds tothat illustrated in FIG. 1B. An example in which the first thin filmtransistor 430 is an enhancement type n-channel transistor and thesecond thin film transistor 431 is a depression type n-channeltransistor is illustrated.

In order to manufacture an enhancement type n-channel transistor and adepression type n-channel transistor over the same substrate, forexample, he first oxide semiconductor layer 405 and the secondsemiconductor layer 407 are formed using different materials or underdifferent conditions. Alternatively, an EDMOS circuit may be formed insuch a manner that gate electrodes are provided over and under the oxidesemiconductor layer to control the threshold value and voltage isapplied to the gate electrodes so that one of the TFTs is normally onwhile the other TFT is normally off.

Embodiment 2

Although the example of the EDMOS circuit is described in Embodiment 1,an equivalent circuit of an EEMOS circuit is illustrated in FIG. 2A inEmbodiment 2. In the equivalent circuit illustrated in FIG. 2A, a drivercircuit can be formed using either a combination of enhancement typen-channel transistors or a combination of an enhancement type n-channeltransistor as a first thin film transistor 460 and a depression typen-channel transistor as a second thin film transistor 461 that is theother one.

It can be said that using the circuit configuration illustrated in FIG.2A in which enhancement type n-channel transistors of the same type arecombined for the driver circuit is preferable, in which case atransistor used for a pixel portion is also formed of an enhancementtype n-channel transistor which is the same type as that used for thedriver circuit, which does not cause an increase in the number ofmanufacturing steps. In addition, FIG. 2B is a top view.

In addition, an example of a manufacturing process of an invertercircuit is illustrated in FIGS. 3A to 3C. Note that a cross sectiontaken along the chain line Y1-Y2 corresponds to FIG. 3C.

A first conductive film is formed over a substrate 440 by a sputteringmethod and the first conductive film is selectively etched using a firstphotomask to form a first gate electrode 441 and a second gate electrode442. Next, a gate insulating layer 443 for covering the first gateelectrode 401 and the second gate electrode 442 is formed by a plasmaCVD method or a sputtering method. The gate insulating layer 443 can beformed to have a single layer or a stacked layer of a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, or a siliconnitride oxide layer by a CVD method, a sputtering method, or the like.Alternatively, the gate insulating layer 443 can be formed of a siliconoxide layer by a CVD method using an organosilane gas. As theorganosilane gas, a silicon-containing compound such astetraethoxysilane (TEOS: chemical formula, Si(OC₂H₅)₄),tetramethylsilane (TMS: chemical formula, Si(CH₃)₄),tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC₂H₅)₃), ortrisdimethylaminosilane (SiH(N(CH₃)₂)₃) can be used.

Next, the gate insulating layer 443 is selectively etched using a secondphotomask to form a contact hole 444 that reaches the second gateelectrode 442. A cross-sectional view of the steps so far corresponds toFIG. 3A.

Next, a second conductive film is formed over the gate insulating layer443 by a sputtering method and the second conductive film is selectivelyetched using a third photomask to form a first wiring 449, a secondwiring 450, and a third wiring 451. The third wiring 451 is directly incontact with the second gate electrode 442 through the contact hole 444.Note that reverse sputtering in which plasma is generated byintroduction of an argon gas is preferably performed to remove dustattached to a surface of the gate insulating layer 443 and a bottomsurface of the contact hole 444. The reverse sputtering is a method inwhich voltage is applied to a substrate side, not to a target side, inan argon atmosphere and plasma is generated so that a substrate surfaceis modified. Note that nitrogen, helium, or the like may be used insteadof an argon atmosphere. Alternatively, the reverse sputtering may beperformed in an argon atmosphere to which oxygen, hydrogen, N₂O, or thelike is added. Further alternatively, the reverse sputtering may beperformed in an argon atmosphere to which Cl₂, CF₄, or the like isadded.

Next, an oxide semiconductor film is formed by a sputtering method.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used as a sputtering power source, a DCsputtering method, and a pulsed DC sputtering method in which a bias isapplied in a pulsed manner. An RF sputtering method is mainly used inthe case of forming an insulating film, and a DC sputtering method ismainly used in the case of forming a metal film.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering method, ora sputtering apparatus used for an ECR sputtering method in which plasmagenerated with use of microwaves is used without using glow discharge.

In the sputtering chamber of this embodiment, any of a variety ofsputtering ways described above is used as appropriate.

Alternatively, as a deposition method, the following are also given: areactive sputtering method in which a target substance and a sputteringgas component are chemically reacted with each other during depositionto form a thin compound film thereof and a bias sputtering method inwhich voltage is also applied to a substrate during deposition.

Note that reverse sputtering in which plasma is generated byintroduction of an argon gas is preferably performed to remove dustattached to the surface of the gate insulating layer 443 and thesurfaces of the first wiring 449, the second wiring 450, and the thirdwiring 451 before the oxide semiconductor film is formed by a sputteringmethod. The reverse sputtering is a method in which voltage is appliedto a substrate side, not to a target side, in an argon atmosphere andplasma is generated so that a substrate surface is modified. Note thatnitrogen, helium, or the like may be used instead of an argonatmosphere. Alternatively, the reverse sputtering may be performed in anargon atmosphere to which oxygen, hydrogen, N₂O, or the like is added.Further alternatively, the reverse sputtering may be performed in anargon atmosphere to which Cl₂, CF₄, or the like is added.

Next, the oxide semiconductor film is selectively etched using a fourthphotomask. When this etching step is finished, a first thin filmtransistor 460 and a second thin film transistor 461 are completed. Across-sectional view of the steps so far corresponds to FIG. 3B.

Next, heat treatment is performed at 200° C. to 600° C. in an airatmosphere or a nitrogen atmosphere. Note that the timing of this heattreatment is not particularly limited and the heat treatment may beperformed anytime as long as it is performed after the formation of theoxide semiconductor film.

Next, a protective layer 452 is formed and the protective layer 452 isselectively etched using a fifth photomask to form a contact hole. Afterthat, a third conductive film is formed. Finally, the third conductivefilm is selectively etched using a sixth photomask to form a connectionwiring 453 that is electrically connected to the second wiring 450. Across-sectional view of the steps so far corresponds to FIG. 3C.

In a light-emitting display device in which a light-emitting element isused, a pixel portion has a plurality of thin film transistors, and thepixel portion also has a contact hole for electrically connecting a gateelectrode of one thin film transistor to a source wiring or a drainwiring of another transistor. This contact portion can be formed usingthe same mask as in the step of forming the contact hole in the gateinsulating layer using the second photomask.

Further, as for a liquid crystal display device or an electronic paper,in a terminal portion for connection to an external terminal such as anFPC, the same mask can be used for a step of forming a contact hole thatreaches a gate wiring and a step of forming a contact hole in a gateinsulating layer using the second photomask.

Embodiment 3

In Embodiment 3, a manufacturing process of a terminal portion and athin film transistor of a pixel portion which can be formed over thesame substrate as the driver circuit described in Embodiment 1 or 2 andwill be described using FIGS. 4A to 4C to FIG. 11.

In FIG. 4A, as a substrate 100 having a light-transmitting property, aglass substrate of barium borosilicate glass, aluminoborosilicate glass,or the like typified by #7059 glass, #1737 glass, or the likemanufactured by Corning, Inc. can be used.

Next, a conductive layer is formed over the entire surface of thesubstrate 100. After that, a first photolithography step is performed toform a resist mask, and unnecessary portions are removed by etching,thereby forming wirings and an electrode (a gate wiring including a gateelectrode layer 101, a capacitor wiring 108, and a first terminal 121).At this time, the etching is performed so that at least end portions ofthe gate electrode layer 101 have a tapered shape. A cross-sectionalview at this stage is illustrated in FIG. 4A. Note that a top view atthis stage corresponds to FIG. 6.

The gate wiring including the gate electrode layer 101, the capacitorwiring 108, and the first terminal 121 of a terminal portion arepreferably formed from a conductive material having low resistance, suchas aluminium (Al) or copper (Cu). However, since use of aluminum alonebrings disadvantages such as low heat resistance and a tendency to becorroded, aluminum is used in combination with a conductive materialhaving heat resistance. As the conductive material having heatresistance, any of the following materials may be used: an elementselected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum(Mo), chromium (Cr), and neodymium (Nd), scandium (Sc), an alloycontaining any of these above elements as a component, an alloycontaining these elements in combination, and a nitride containing anyof these above elements as a component.

Next, a gate insulating layer 102 is formed over the entire surface ofthe gate electrode layer 101. The gate insulating layer 102 is formed toa thickness of 50 to 250 nm by a PCVD method, a sputtering method, orthe like.

For example, for the gate insulating layer 102, a 100-nm-thick siliconoxide film is formed by a PCVD method or a sputtering method. Needlessto say, the gate insulating layer 102 is not limited to such a film andmay be a single layer or a stack of any other types of insulating filmssuch as a silicon oxynitride film, a silicon nitride film, an aluminumoxide film, and a tantalum oxide film.

Next, a second photolithography step is performed to form a resist maskand unnecessary portions are removed by etching, thereby forming acontact hole which includes the same material as that of the gateelectrode layer and reaches the wiring or the electrode. For example, acontact hole is formed when a thin film transistor whose gate electrodeis in direct contact with the source or drain electrode in the drivingcircuit is formed, or when a terminal that is electrically connected toa gate wiring of a terminal portion is formed.

Next, a conductive film formed of a metal material is formed by asputtering method or a vacuum evaporation method. Here, the conductivefilm has a three-layer structure of a Ti film, an aluminum filmcontaining Nd, and a Ti film. As a material of the conductive film, anelement selected from Al, Cr, Ta, Ti, Mo, and W; an alloy containing theabove element as a component; and an alloy film in which the aboveelements are combined are given. Alternatively, the conductive film mayhave a two-later structure, and in such a case, a titanium film may bestacked over an aluminum film. Further alternatively, the conductivefilm may have a single-layer structure of an aluminum film containingsilicon or a titanium film.

Next, a third photolithography step is performed to form a resist maskand unnecessary portions are removed by etching, thereby forming sourceor drain electrode layers 105 a and 105 b and a connection electrode120. Wet etching or dry etching is used as an etching method at thistime. Here, the Ti film is etched using an ammonia hydrogen peroxidemixture (with the ratio of hydrogen peroxide to ammonia and water being5:2:2) as an etchant and the aluminum film containing Nd is etched usinga mixed solution of phosphoric acid, acetic acid, and nitric acid as anetchant. A conductive film in which the Ti film, the Al—Nd film, and theTi film are sequentially stacked is etched by this etching to form thesource or drain electrode layers 105 a and 105 b. A cross-sectional viewat this stage is illustrated in FIG. 4B. Note that FIG. 7 is a top viewat this stage.

In the terminal portion, the connection electrode 120 is directlyconnected to the first terminal 121 through the contact hole formed inthe gate insulating layer. Note that, although not illustrated here, asource wiring or a drain wiring of a thin film transistor of a drivercircuit is directly connected to the gate electrode through the sameprocess as the above.

Next, plasma treatment is performed after the resist mask is removed. Across-sectional view at this stage is illustrated in FIG. 4C. Here, byreverse sputtering in which plasma is generated by an RF power supply byintroduction of an argon gas, plasma treatment is performed on theexposed gate insulating layer.

Next, after the plasma treatment, an oxide semiconductor film is formedwithout exposure to air. Formation of the oxide semiconductor filmwithout exposure to air after the plasma treatment is effective inpreventing dust and moisture from attaching to the interface between thegate insulating layer and the oxide semiconductor film. In Embodiment 3,the oxide semiconductor film is formed in an argon or oxygen atmosphereusing an oxide semiconductor target containing In, Ga, and Zn and havinga diameter of 8 inches (In₂O₃:Ga₂O₃:ZnO=1:1:1), with the distancebetween the substrate and the target set to 170 mm, under a pressure of0.4 Pa, and with a direct-current (DC) power source of 0.5 kW. Note thatit is preferable to use a pulsed direct-current (DC) power source withwhich dust can be reduced and thickness distribution can be evened. Theoxide semiconductor film has a thickness of 5 nm to 200 nm In Embodiment3, the thickness of the oxide semiconductor film is 100 nm.

The oxide semiconductor film may be formed in the same chamber as thatin which reverse sputtering has been performed, or may be formed in achamber different from that in which reverse sputtering has beenperformed as long as it can be formed without exposure to air.

Next, a fourth photolithography step is performed to form a resist mask,and an unnecessary portion is removed by etching to form an oxidesemiconductor layer 103. Here, the unnecessary portion is removed by wetetching using ITO-07N (KANTO CHEMICAL CO., INC.) to form the oxidesemiconductor layer 103. Note that the etching here may be dry etchingwithout being limited to wet etching. After that, the resist mask isremoved.

Next, heat treatment is preferably performed at 200° C. to 600° C.,typically, 300° C. to 500° C. For example, heat treatment is performedin a nitrogen atmosphere in a furnace at 350° C. for 1 hour. Through theabove steps, a thin film transistor 170 in which the oxide semiconductorlayer 103 serves as a channel formation region can be manufactured. Across-sectional view at this point is illustrated in FIG. 5A. A top viewat this stage corresponds to FIG. 8. In addition, the cross-sectionalview of FIG. 5A corresponds to FIG. 3B illustrating the manufacturingprocess of the driver circuit described in Embodiment 2. Note that thetiming of the heat treatment is not particularly limited as long as itis after the formation of the oxide semiconductor film. The heattreatment may be performed, for example, after formation of a protectiveinsulating film.

Next, a protective insulating film 107 for covering the oxidesemiconductor layer 103 is formed. For the protective insulating film107, a silicon nitride film, a silicon oxide film, a silicon oxynitridefilm, an aluminum oxide film, a tantalum oxide film, or the like whichis obtained by a sputtering method or the like can be used. In addition,oxygen radical treatment is preferably performed on a surface of theoxide semiconductor layer 103 before the protective insulating film 107is formed. Plasma treatment or reverse sputtering may be performed asthe oxygen radical treatment performed on the oxide semiconductor layer103. The reverse sputtering is a method in which voltage is applied to asubstrate side, not to a target side, in an oxygen atmosphere or anatmosphere containing oxygen and argon and plasma is generated so that asubstrate surface is modified. By the oxygen radical treatment performedon the surface of the oxide semiconductor layer 103, a threshold voltageof the thin film transistor 170 can be positive, whereby a so-callednormally-off switching element can be realized. It is desirable, for adisplay device, that a channel be formed under such a condition that thegate voltage of the thin film transistor is a threshold voltage that ispositive and as close to 0 V as possible. Note that if the thresholdvoltage of the thin film transistor is negative, the thin filmtransistor is likely to be a so-called normally-on transistor in whichcurrent flows between a source electrode and a drain electrode even at agate voltage of 0 V.

Next, a fifth photolithography step is performed to form a resist mask,and the protective insulating film 107 is etched to form a contact hole125 which reaches the drain electrode layer 105 b. After that, theresist mask is removed. In addition, by the etching here, a contact hole127 which reaches the second terminal 122 is also formed. Note that inorder to reduce the number of masks, the gate insulating layer ispreferably etched using the same resist mask so that a contact hole 126which reaches the gate electrode is formed using the same resist mask. Across-sectional view at this stage is illustrated in FIG. 5B.

Next, a transparent conductive film is formed over the protectiveinsulating film 107. The transparent conductive film is formed usingindium oxide (In₂O₃), an alloy of indium oxide and tin oxide(In₂O₃—SnO₂, abbreviated as ITO), or the like by a sputtering method, avacuum evaporation method, or the like. Etching treatment of such amaterial is performed with a hydrochloric acid based solution. Instead,because a residue tends to be generated particularly in etching of ITO,an alloy of indium oxide and zinc oxide (In₂O₃—ZnO) may be used in orderto improve etching proccessability.

Next, a sixth photolithography step is performed to form a resist maskand unnecessary portions are removed by etching, thereby forming a pixelelectrode layer 110.

Further, in this sixth photolithography step, a storage capacitor isformed with the capacitor wiring 108 and the pixel electrode layer 110,in which the gate insulating layer 102 and the protective insulatingfilm 107 in the capacitor portion are used as a dielectric.

In addition, in this seventh photolithography step, upper portions ofthe first terminal and the second terminal are covered with the resistmask so that transparent conductive films 128 and 129 are left in theterminal portion. The transparent conductive films 128 and 129 serve aselectrodes or wirings that are used for connection with an FPC. Thetransparent conductive film 128 formed over the connection electrode 120that is directly connected to the first terminal 121 serves as aterminal electrode for connection which functions as an input terminalfor the gate wiring. The transparent conductive film 129 formed over thesecond terminal 122 serves as a terminal electrode for connection whichfunctions as an input terminal for the source wiring.

Next, the resist mask is removed, and a cross-sectional view at thisstage is illustrated in FIG. 5C. Note that a top view at this stagecorresponds to FIG. 9. In addition, the cross-sectional view of FIG. 5Ccorresponds to FIG. 3C illustrating the manufacturing step of the drivercircuit described in Embodiment 2.

Further, FIGS. 10A and 10C are a cross-sectional view of a gate wiringterminal portion at this stage and a top view thereof, respectively.FIG. 10A is a cross-sectional view taken along the line C1-C2 of FIG.10C. In FIG. 10A, a transparent conductive film 155 formed over aprotective insulating film 154 is a connection terminal electrode whichfunctions as an input terminal Furthermore, in FIG. 10A, in the terminalportion, the first terminal 151 formed from the same material as thegate wiring and the connection electrode 153 formed from the samematerial as the source wiring are overlapped with each other with thegate insulating layer 152 therebetween, and the first terminal 151 andthe connection electrode 153 are in direct contact with each otherthrough a contact hole provided in the gate insulating layer 152 to formconduction therebetween. In addition, the connection electrode 153 andthe transparent conductive film 155 are in direct contact with eachother through a contact hole provided in the protective insulating film154 to form conduction therebetween.

Further, FIGS. 10B and 10D are a cross-sectional view of a source wiringterminal portion at this stage and a top view thereof, respectively. Inaddition, FIG. 10B corresponds to a cross-sectional view taken along theline G1-G2 in FIG. 10D. In FIG. 10B, the transparent conductive film 155formed over the protective insulating film 154 is a connection terminalelectrode which functions as an input terminal. Furthermore, in FIG.10B, in the terminal portion, an electrode 156 formed from the samematerial as the gate wiring is located below and overlapped with thesecond terminal 150, which is electrically connected to the sourcewiring, with the gate insulating layer 152 interposed therebetween. Theelectrode 156 is not electrically connected to the second terminal 150.When the electrode 156 is set to, for example, floating, GND, or 0 Vsuch that the potential the electrode 156 is different from thepotential of the second terminal 150, a capacitor for preventing noiseor static electricity can be formed. In addition, the second terminal150 is electrically connected to the transparent conductive film 155with the protective insulating film 154 interposed therebetween.

A plurality of gate wirings, source wirings, and capacitor wirings areprovided depending on the pixel density. Also in the terminal portion,the first terminal at the same potential as the gate wiring, the secondterminal at the same potential as the source wiring, the third terminalat the same potential as the capacitor wiring, and the like are eacharranged in plurality. There is no particular limitation on the numberof each of the terminals, and the number of the terminals may bedetermined by a practitioner as appropriate.

Through these six photolithography steps, a pixel portion including thebottom-gate thin film transistor 170, and the storage capacitor can becompleted using the six photomasks. When these pixel thin filmtransistor portion and storage capacitor are arranged in a matrixcorresponding to respective pixels, a pixel portion can be formed andone of the substrates for manufacturing an active matrix display devicecan be obtained. In this specification, such a substrate is referred toas an active matrix substrate for convenience.

When an active matrix liquid crystal display device is manufactured, anactive matrix substrate and a counter substrate provided with a counterelectrode are bonded to each other with a liquid crystal layerinterposed therebetween. Note that a common electrode electricallyconnected to the counter electrode on the counter substrate is providedover the active matrix substrate, and a fourth terminal electricallyconnected to the common electrode is provided in the terminal portion.This fourth terminal is provided so that the common electrode is fixedto a predetermined potential such as GND or 0 V.

Further, an embodiment of the present invention is not limited to apixel structure in FIG. 9, and an example of a top view different fromFIG. 9 is illustrated in FIG. 11. FIG. 11 illustrates an example inwhich a capacitor wiring is not provided and a storage capacitor isformed with a pixel electrode and a gate wiring of an adjacent pixelwhich are overlapped with each other with a protective insulating filmand a gate insulating layer interposed therebetween. In this case, thecapacitor wiring and the third terminal connected to the capacitorwiring can be omitted. Note that in FIG. 11, portions similar to thosein FIG. 9 are denoted by the same reference numerals.

In an active matrix liquid crystal display device, pixel electrodesarranged in a matrix are driven to form a display pattern on a screen.Specifically, a voltage is applied between a selected pixel electrodeand a counter electrode corresponding to the pixel electrode, so that aliquid crystal layer provided between the pixel electrode and thecounter electrode is optically modulated and this optical modulation isrecognized as a display pattern by an observer.

In displaying moving images, a liquid crystal display device has aproblem that a long response time of liquid crystal molecules themselvescauses afterimages or blurring of moving images. In order to improve themoving-image characteristics of a liquid crystal display device, adriving method called black insertion is employed in which black isdisplayed on the whole screen every other frame period.

Further, there is another driving technique which is so-calleddouble-frame rate driving. In the double-frame rate driving, a verticalcycle is set 1.5 times as much as a normal vertical cycle or more(preferably 2 times or more), whereby moving image characteristics areimproved.

Further alternatively, in order to improve the moving-imagecharacteristics of a liquid crystal display device, a driving method maybe employed in which a plurality of LEDs (light-emitting diodes) or aplurality of EL light sources are used to form a surface light source asa backlight, and each light source of the surface light source isindependently driven in a pulsed manner in one frame period. As thesurface light source, three or more kinds of LEDs may be used and an LEDemitting white light may be used. Since a plurality of LEDs can becontrolled independently, the light emission timing of LEDs can besynchronized with the timing at which a liquid crystal layer isoptically modulated. According to this driving method, LEDs can bepartly turned off; therefore, an effect of reducing power consumptioncan be obtained particularly in the case of displaying an image having alarge part on which black is displayed.

By combining these driving methods, the display characteristics of aliquid crystal display device, such as moving-image characteristics, canbe improved as compared to those of conventional liquid crystal displaydevices.

The n-channel transistor obtained in Embodiment 3 includes anIn—Ga—Zn—O-based non-single-crystal film in a channel formation regionand has good dynamic characteristics. Thus, these driving methods can beapplied in combination to the n-channel transistor of this embodiment.

When a light-emitting display device is manufactured, one electrode(also referred to as a cathode) of an organic light-emitting element isset to a low power supply potential such as GND or 0 V; therefore, aterminal portion is provided with a fourth terminal for setting thecathode to a low power supply potential such as GND or 0 V. In addition,when a light-emitting display device is manufactured, a power supplyline is provided in addition to a source wiring and a gate wiring.Therefore, a terminal portion is provided with a fifth terminalelectrically connected to the power supply line.

Embodiment 3 can be freely combined with Embodiment 1 or 2.

Embodiment 4

In Embodiment 4, an example of an electronic paper will be described asa semiconductor device of an embodiment of the present invention.

FIG. 12 illustrates an active matrix electronic paper as an example of asemiconductor device to which an embodiment of the present invention isapplied. A thin film transistor 581 used for the semiconductor devicecan be manufactured in a manner similar to that of the thin filmtransistor 170 described in Embodiment 3 and is a thin film transistorwith high electrical characteristics including an oxide semiconductorlayer over a gate insulating layer, a source electrode layer, and adrain electrode layer.

The electronic paper in FIG. 12 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black or whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and apotential difference is generated between the first electrode layer andthe second electrode layer to control orientation of the sphericalparticles, so that display is performed.

The thin film transistor 581 is a thin film transistor with a bottomgate structure, and a source or drain electrode layer thereof is incontact with a first electrode layer 587 through an opening formed ininsulating layers 583, 584, and 585, whereby the thin film transistor581 is electrically connected to the first electrode layer 587. Betweenthe first electrode layer 587 and a second electrode layer 588,spherical particles 589 each having a black region 590 a, a white region590 b, and a cavity 594 around the regions which is filled with liquidare provided. A space around the spherical particles 589 is filled witha filler 595 such as a resin (see FIG. 12). In Embodiment 8, the firstelectrode layer 587 corresponds to a pixel electrode, and the secondelectrode layer 588 corresponds to a common electrode.

Instead of the twisting ball, an electrophoretic element can also beused. A microcapsule having a diameter of about 10 to 200 μm in whichtransparent liquid, positively-charged white microparticles, andnegatively-charged black microparticles are encapsulated, is used. Inthe microcapsule which is provided between the first electrode layer andthe second electrode layer, when an electric field is applied betweenthe first electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides fromeach other, so that white or black can be displayed. A display elementusing this principle is an electrophoretic display element and isgenerally called an electronic paper. The electrophoretic displayelement has higher reflectance than a liquid crystal display element,and thus, an auxiliary light is unnecessary, power consumption is low,and a display portion can be recognized in a dim place. In addition,even when power is not supplied to the display portion, an image whichhas been displayed once can be maintained. Accordingly, a displayedimage can be stored even if a semiconductor device having a displayfunction (which may be referred to simply as a display device or asemiconductor device provided with a display device) is distanced froman electric wave source.

Through this process, a highly reliable electronic paper as asemiconductor device can be manufactured.

Embodiment 4 can be implemented in appropriate combination with thedriver circuit or the pixel portion described in any one of Embodiments1 to 3.

Embodiment 5

In Embodiment 5, an example will be described below, in which at leastpart of a driver circuit and a thin film transistor arranged in a pixelportion are formed over the same substrate in a display device which isone example of a semiconductor device of an embodiment of the presentinvention.

The thin film transistor to be arranged in the pixel portion is formedaccording to Embodiment 3. Further, the thin film transistor 170described in Embodiment 3 is an n-channel TFT, and thus a part of adriver circuit that can include an n-channel TFT among driver circuitsis formed over the same substrate as the thin film transistor of thepixel portion.

FIG. 13A is an example of a block diagram of an active matrix liquidcrystal display device which is an example of a semiconductor device ofan embodiment of the present invention. The display device illustratedin FIG. 13A includes, over a substrate 5300, a pixel portion 5301including a plurality of pixels that are each provided with a displayelement; a gate line driver circuit 5302 that selects a pixel; and asource line driver circuit 5303 that controls a video signal input tothe selected pixel.

In addition, the thin film transistor 170 described in Embodiment 3 isan n-channel TFT, and a source line driver circuit including then-channel TFT is described with reference to FIG. 14.

The source line driver circuit illustrated in FIG. 14 includes a driverIC 5601, switch groups 5602_1 to 5602_M, a first wiring 5611, a secondwiring 5612, a third wiring 5613, and wirings 5621_1 to 5621_M. Each ofthe switch groups 5602_1 to 5602_M includes a first thin film transistor5603 a, a second thin film transistor 5603 b, and a third thin filmtransistor 5603 c.

The pixel portion 5301 is connected to the source line driver circuit5303 by a plurality of signal lines S1 to Sm (not illustrated) thatextend in a column direction from the source line driver circuit 5303,and to the gate line driver circuit 5302 by a plurality of scan lines G1to Gn (not illustrated) that extend in a row direction from the gateline driver circuit 5302. The pixel portion 5301 includes a plurality ofpixels (not illustrated) arranged in matrix so as to correspond to thesignal lines S1 to Sm and the scan lines G1 to Gn. Each pixel isconnected to a signal line Sj (one of the signal lines S1 to Sm) and ascan line Gj (one of the scan lines G1 to Gn).

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M.Each of the switch groups 5602_1 to 5602_M is connected to the firstwiring 5611, the second wiring 5612, and the third wiring 5613, and thewirings 5621_1 to 5621_M are connected to the switch groups 5602_1 to5602_M, respectively. Each of the wirings 5621_1 to 5621_M is connectedto three signal lines via the first thin film transistor 5603 a, thesecond thin film transistor 5603 b, and the third thin film transistor5603 c. For example, the wiring 5621_J of the J-th column (one of thewirings 5621_1 to 5621_M) is connected to a signal line Sj−1, a signalline Sj, and a signal line Sj+1 via the first thin film transistor 5603a, the second thin film transistor 5603 b, and the third thin filmtransistor 5603 c which are included in the switch group 5602_J.

Note that a signal is inputted to each of the first wiring 5611, thesecond wiring 5612, and the third wiring 5613.

Note that the driver IC 5601 is preferably formed over a singlecrystalline substrate. The switch groups 5602_1 to 5602_M are preferablyformed over the same substrate as the pixel portion is. Therefore, thedriver IC 5601 and the switch groups 5602_1 to 5602_M are preferablyconnected through an FPC or the like.

Next, operation of the source line driver circuit illustrated in FIG. 14is described with reference to a timing chart in FIG. 15. The timingchart in FIG. 15 illustrates a case where the scan line Gi of the i-throw is selected. A selection period of the scan line Gi of the i-th rowis divided into a first sub-selection period T1, a second sub-selectionperiod T2, and a third sub-selection period T3. In addition, the sourceline driver circuit in FIG. 14 operates similarly to that in FIG. 15even when a scan line of another row is selected.

Note that the timing chart in FIG. 15 shows a case where the wiring5621_J of the J-th column is connected to the signal line Sj−1, thesignal line Sj, and the signal line Sj+1 via the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c.

The timing chart in FIG. 15 shows timing at which the scan line Gi ofthe i-th row is selected, timing 5703 a of on/off of the first thin filmtransistor 5603 a, timing 5703 b of on/off of the second thin filmtransistor 5603 b, timing 5703 c of on/off of the third thin filmtransistor 5603 c, and a signal 5721_J input to the wiring 5621_J of theJ-th column

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinput to the wirings 5621_1 to 5621_M. For example, a video signal inputto the wiring 5621_J in the first sub-selection period T1 is input tothe signal line Sj−1, a video signal input to the wiring 5621_J in thesecond sub-selection period T2 is input to the signal line Sj, and avideo signal input to the wiring 5621_J in the third sub-selectionperiod T3 is input to the signal line Sj+1. In addition, the videosignals input to the wiring 5621_J in the first sub-selection period T1,the second sub-selection period T2, and the third sub-selection periodT3 are denoted by Data_j−1, Data_j, and Data_j+1

As illustrated in FIG. 15, in the first sub-selection period T1, thefirst thin film transistor 5603 a is turned on, and the second thin filmtransistor 5603 b and the third thin film transistor 5603 c are turnedoff. At this time, Data_j−1 input to the wiring 5621_J is input to thesignal line Sj−1 via the first thin film transistor 5603 a. In thesecond sub-selection period T2, the second thin film transistor 5603 bis turned on, and the first thin film transistor 5603 a and the thirdthin film transistor 5603 c are turned off. At this time, Data_j inputto the wiring 5621_J is input to the signal line Sj via the second thinfilm transistor 5603 b. In the third sub-selection period T3, the thirdthin film transistor 5603 c is turned on, and the first thin filmtransistor 5603 a and the second thin film transistor 5603 b are turnedoff. At this time, Data_j+1 input to the wiring 5621_J is input to thesignal line Sj+1 via the third thin film transistor 5603 c.

As described above, in the source line driver circuit in FIG. 14, bydividing one gate selection period into three, video signals can beinput to three signal lines from one wiring 5621 in one gate selectionperiod. Therefore, in the source line driver circuit in FIG. 14, thenumber of connections between the substrate provided with the driver IC5601 and the substrate provided with the pixel portion can beapproximately ⅓ of the number of signal lines. The number of connectionsis reduced to approximately ⅓ of the number of the signal lines, so thatreliability, yield, etc., of the source line driver circuit in FIG. 14can be improved.

Note that there are no particular limitations on the arrangement, thenumber, a driving method, and the like of the thin film transistors, aslong as one gate selection period is divided into a plurality ofsub-selection periods and video signals are input to a plurality ofsignal lines from one wiring in the respective sub-selection periods asillustrated in FIG. 14.

For example, when video signals are input to three or more signal linesfrom one wiring in each of three or more sub-selection periods, it isonly necessary to add a thin film transistor and a wiring forcontrolling the thin film transistor. Note that when one gate selectionperiod is divided into four or more sub-selection periods, onesub-selection period becomes short. Therefore, one gate selection periodis preferably divided into two or three sub-selection periods.

As another example, one gate selection period may be divided into aprecharge period Tp, the first sub-selection period T1, the secondsub-selection period T2, and the third sub-selection period T3 as shownin a timing chart in FIG. 16. The timing chart in FIG. 16 shows timingat which the scan line Gi of the i-th row is selected, timing 5803 a ofon/off of the first thin film transistor 5603 a, timing 5803 b of on/offof the second thin film transistor 5603 b, timing 5803 c of on/off ofthe third thin film transistor 5603 c, and a signal 5821_J input to thewiring 5621_J of the J-th column. As shown in FIG. 16, the first thinfilm transistor 5603 a, the second thin film transistor 5603 b, and thethird thin film transistor 5603 c are tuned on in the precharge periodTp. At this time, precharge voltage Vp input to the wiring 5621_J isinput to each of the signal line Sj−1, the signal line Sj, and thesignal line Sj+1 via the first thin film transistor 5603 a, the secondthin film transistor 5603 b, and the third thin film transistor 5603 c.In the first sub-selection period T1, the first thin film transistor5603 a is turned on, and the second thin film transistor 5603 b and thethird thin film transistor 5603 c are turned off. At this time, Data_j−1input to the wiring 5621_J is input to the signal line Sj−1 via thefirst thin film transistor 5603 a. In the second sub-selection periodT2, the second thin film transistor 5603 b is turned on, and the firstthin film transistor 5603 a and the third thin film transistor 5603 care turned off. At this time, Data_j input to the wiring 5621_J is inputto the signal line Sj via the second thin film transistor 5603 b. In thethird sub-selection period T3, the third thin film transistor 5603 c isturned on, and the first thin film transistor 5603 a and the second thinfilm transistor 5603 b are turned off. At this time, Data_j+1 input tothe wiring 5621_J is input to the signal line Sj+1 via the third thinfilm transistor 5603 c.

As described above, in the source line driver circuit in FIG. 14 towhich the timing chart in FIG. 16 is applied, the video signal can bewritten to the pixel at high speed because the signal line can beprecharged by providing a precharge selection period before asub-selection period. Note that portions in FIG. 16 which are similar tothose of FIG. 15 are denoted by common reference numerals and detaileddescription of the same portions and portions which have similarfunctions is omitted.

Further, a structure of a gate line driver circuit is described. Thegate line driver circuit includes a shift register or a buffer.Additionally, the gate line driver circuit may include a level shifteror may include only a shift register in some cases. In the gate linedriver circuit, when the clock signal (CLK) and the start pulse signal(SP) are input to the shift register, a selection signal is produced.The selection signal produced is buffered and amplified by the buffer,and the resulting signal is supplied to a corresponding scan line. Gateelectrodes of transistors in pixels of one line are connected to thescan line. Further, since the transistors in the pixels of one line haveto be turned on at the same time, a buffer which can feed a largecurrent is used.

One mode of a shift register which is used for a part of a gate linedriver circuit is described with reference to FIG. 17 and FIG. 18.

FIG. 17 illustrates a circuit configuration of the shift register. Theshift register illustrated in FIG. 17 includes a plurality of flip-flops5701_1 to 5701 _(—) n. The shift register is operated with input of afirst clock signal, a second clock signal, a start pulse signal, and areset signal.

Connection relations of the shift register in FIG. 17 are described. Inthe i-th stage flip-flop 5701 _(—) i (one of the flip-flops 5701_1 to5701 _(—) n) in the shift register of FIG. 17, a first wiring 5501illustrated in FIG. 18 is connected to a seventh wiring 5717_i−1; asecond wiring 5502 illustrated in FIG. 18 is connected to a seventhwiring 5717_i+1; a third wiring 5503 illustrated in FIG. 18 is connectedto a seventh wiring 5717 _(—) i; and a sixth wiring 5506 illustrated inFIG. 18 is connected to a fifth wiring 5715.

Further, a fourth wiring 5504 illustrated in FIG. 18 is connected to asecond wiring 5712 in flip-flops of odd-numbered stages, and isconnected to a third wiring 5713 in flip-flops of even-numbered stages.A fifth wiring 5505 illustrated in FIG. 18 is connected to a fourthwiring 5714.

Note that the first wiring 5501 of the first stage flip-flop 5701_1illustrated in FIG. 18 is connected to a first wiring 5711. Moreover,the second wiring 5502 of the n-th stage flip-flop 5701 _(—) nillustrated in FIG. 18 is connected to a sixth wiring 5716.

Note that the first wiring 5711, the second wiring 5712, the thirdwiring 5713, and the sixth wiring 5716 may be referred to as a firstsignal line, a second signal line, a third signal line, and a fourthsignal line, respectively. The fourth wiring 5714 and the fifth wiring5715 may be referred to as a first power supply line and a second powersupply line, respectively.

Next, FIG. 18 illustrates details of the flip-flop illustrated in FIG.17. A flip-flop illustrated in FIG. 18 includes a first thin filmtransistor 5571, a second thin film transistor 5572, a third thin filmtransistor 5573, a fourth thin film transistor 5574, a fifth thin filmtransistor 5575, a sixth thin film transistor 5576, a seventh thin filmtransistor 5577, and an eighth thin film transistor 5578. Each of thefirst thin film transistor 5571, the second thin film transistor 5572,the third thin film transistor 5573, the fourth thin film transistor5574, the fifth thin film transistor 5575, the sixth thin filmtransistor 5576, the seventh thin film transistor 5577, and the eighththin film transistor 5578 is an n-channel transistor and is turned onwhen the gate-source voltage (V_(gs)) exceeds the threshold voltage(V_(th)).

In FIG. 18, a gate electrode of the third thin film transistor 5573 iselectrically connected to a power supply line. In addition, a circuit inwhich the third thin film transistor 5573 and the fourth thin filmtransistor 5574 are connected to each other (a circuit surrounded by achain line in FIG. 18) can be said to correspond to the circuitconfiguration illustrated in FIG. 2A. Although all the thin filmtransistors here are enhancement type n-channel transistors, there is noparticular limitation thereto, and a driver circuit can be driven evenif the third thin film transistor 5573 is a depression type n-channeltransistor.

Next, connections of the flip-flop illustrated in FIG. 18 are describedbelow.

A first electrode (one of a source electrode and a drain electrode) ofthe first thin film transistor 5571 is connected to the fourth wiring5504. A second electrode (the other of the source electrode and thedrain electrode) of the first thin film transistor 5571 is connected tothe third wiring 5503.

A first electrode of the second thin film transistor 5572 is connectedto the sixth wiring 5506. A second electrode of the second thin filmtransistor 5572 is connected to the third wiring 5503.

A first electrode of the third thin film transistor 5573 is connected tothe fifth wiring 5505. A second electrode of the third thin filmtransistor 5573 is connected to a gate electrode of the second thin filmtransistor 5572. A gate electrode of the third thin film transistor 5573is connected to the fifth wiring 5505.

A first electrode of the fourth thin film transistor 5574 is connectedto the sixth wiring 5506. A second electrode of the fourth thin filmtransistor 5574 is connected to the gate electrode of the second thinfilm transistor 5572. A gate electrode of the fourth thin filmtransistor 5574 is connected to a gate electrode of the first thin filmtransistor 5571.

A first electrode of the fifth thin film transistor 5575 is connected tothe fifth wiring 5505. A second electrode of the fifth thin filmtransistor 5575 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the fifth thin film transistor5575 is connected to the first wiring 5501.

A first electrode of the sixth thin film transistor 5576 is connected tothe sixth wiring 5506. A second electrode of the sixth thin filmtransistor 5576 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the sixth thin film transistor5576 is connected to the gate electrode of the second thin filmtransistor 5572.

A first electrode of the seventh thin film transistor 5577 is connectedto the sixth wiring 5506. A second electrode of the seventh thin filmtransistor 5577 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the seventh thin filmtransistor 5577 is connected to the second wiring 5502. A firstelectrode of the eighth thin film transistor 5578 is connected to thesixth wiring 5506. A second electrode of the eighth thin film transistor5578 is connected to the gate electrode of the second thin filmtransistor 5572. A gate electrode of the eighth thin film transistor5578 is connected to the first wiring 5501.

Note that the points at which the gate electrode of the first thin filmtransistor 5571, the gate electrode of the fourth thin film transistor5574, the second electrode of the fifth thin film transistor 5575, thesecond electrode of the sixth thin film transistor 5576, and the secondelectrode of the seventh thin film transistor 5577 are connected areeach referred to as a node 5543. The points at which the gate electrodeof the second thin film transistor 5572, the second electrode of thethird thin film transistor 5573, the second electrode of the fourth thinfilm transistor 5574, the gate electrode of the sixth thin filmtransistor 5576, and the second electrode of the eighth thin filmtransistor 5578 are connected are each referred to as a node 5544.

Note that the first wiring 5501, the second wiring 5502, the thirdwiring 5503, and the fourth wiring 5504 may be referred to as a firstsignal line, a second signal line, a third signal line, and a fourthsignal line, respectively. The fifth wiring 5505 and the sixth wiring5506 may be referred to as a first power supply line and a second powersupply line, respectively.

In addition, the source line driver circuit and the gate line drivercircuit can be formed using only the n-channel TFTs described inEmbodiment 3. The n-channel TFT described in Embodiment 3 has a highmobility, and thus a driving frequency of a driver circuit can beincreased. Further, parasitic capacitance is reduced by the source ordrain region which is an oxygen-deficient oxide semiconductor layercontaining indium, gallium, and zinc; thus, the n-channel TFT describedin Embodiment 3 has high frequency characteristics. For example, a gateline driver circuit using the n-channel TFT described in Embodiment 3can operate at high speed, and thus a frame frequency can be increasedand insertion of black images can be realized.

In addition, when the channel width of the transistor in the gate linedriver circuit is increased or a plurality of gate line driver circuitsare provided, for example, higher frame frequency can be realized. Whena plurality of gate line driver circuits are provided, a gate linedriver circuit for driving scan lines of even-numbered rows is providedon one side and a gate line driver circuit for driving scan lines ofodd-numbered rows is provided on the opposite side; thus, increase inframe frequency can be realized. In addition, inputting a signal is tothe same scan line by a plurality of gate line driver circuits isadvantageous for increase in size of a display device.

Further, when an active matrix light-emitting display device which is anexample of a semiconductor device of an embodiment of the presentinvention is manufactured, a plurality of thin film transistors arearranged in at least one pixel, and thus a plurality of gate line drivercircuits are preferably arranged. FIG. 13B is a block diagramillustrating an example of an active matrix light-emitting displaydevice.

The light-emitting display device illustrated in FIG. 13B includes, overa substrate 5400, a pixel portion 5401 having a plurality of pixels eachprovided with a display element, a first gate line driver circuit 5402and a second gate line driver circuit 5404 that select a pixel, and asource line driver circuit 5403 that controls input of a video signal tothe selected pixel.

When the video signal input to a pixel of the light-emitting displaydevice illustrated in FIG. 13B is a digital signal, a pixel is in alight-emitting state or in a non-light-emitting state by switching ofON/OFF of a transistor. Thus, grayscale can be displayed using an arearatio grayscale method or a time ratio grayscale method. An area ratiograyscale method refers to a driving method by which one pixel isdivided into a plurality of subpixels and the respective subpixels aredriven independently based on video signals so that grayscale isdisplayed. A time ratio grayscale method refers to a driving method bywhich a period during which a pixel is in a light-emitting state iscontrolled so that grayscale is displayed.

Since the response speed of light-emitting elements is higher than thatof liquid crystal elements or the like, the light-emitting elements aremore suitable for a time ratio grayscale method than liquid-crystaldisplay elements. Specifically, in the case of displaying with a timegray scale method, one frame period is divided into a plurality ofsubframe periods. Then, in accordance with video signals, thelight-emitting element in the pixel is set in a light-emitting state orin a non-light-emitting state during each subframe period. By dividingone frame into a plurality of subframes, the total length of time, inwhich pixels actually emit light in one frame period, can be controlledwith video signals so that gray scales are displayed.

In the example of the light-emitting display device illustrated in FIG.13B, in a case where two switching TFTs are arranged in one pixel, thefirst gate line driver circuit 5402 generates a signal which is input toa first scan line serving as a gate wiring of one of the switching TFTs,and the second gate line driver circuit 5404 generates a signal which isinput to a second scan line serving as a gate wiring of the otherswitching TFT; however, one gate line driver circuit may generate boththe signal which is input to the first scan line and the signal which isinput to the second scan line. In addition, for example, there is apossibility that a plurality of the scan lines used for controlling theoperation of the switching element are provided in each pixel, dependingon the number of switching TFTs included in one element. In that case,one gate line driver circuit may generate all signals that are input tothe plurality of first scan lines, or a plurality of gate line drivercircuits may generate signals that are input to the plurality of firstscan lines.

In addition, also in the light-emitting display device, a part of thedriver circuit that can include n-channel TFTs among driver circuits canbe formed over the same substrate as the thin film transistors of thepixel portion. Alternatively, the source line driver circuit and thegate line driver circuit can be formed using only the n-channel TFTsdescribed in Embodiment 3.

Moreover, the above-described driver circuit can be used for anelectronic paper that drives electronic ink using an elementelectrically connected to a switching element, without being limited toapplications to a liquid crystal display device or a light-emittingdisplay device. The electronic paper is also referred to as anelectrophoretic display device (electrophoretic display) and hasadvantages in that it has the same level of readability as plain paper,it has lower power consumption than other display devices, and it can bemade thin and lightweight.

Electrophoretic displays can have various modes. Electrophoreticdisplays contain a plurality of microcapsules dispersed in a solvent ora solute, each microcapsule containing first particles which arepositively charged and second particles which are negatively charged. Byapplying an electric field to the microcapsules, the particles in themicrocapsules are moved in opposite directions to each other and onlythe color of the particles concentrated on one side is exhibited. Notethat the first particles and the second particles each contain pigmentand do not move without an electric field. Moreover, the colors of thefirst particles and the second particles are different from each other(the colors include colorless or achroma).

In this way, an electrophoretic display is a display that utilizes aso-called dielectrophoretic effect by which a substance that has a highdielectric constant moves to a high-electric field region. Anelectrophoretic display does not need to have a polarizer and a countersubstrate, which are required in a liquid crystal display device, andboth the thickness and weight of the electrophoretic display device canbe a half of those of a liquid crystal display device.

A solution in which the aforementioned microcapsules are dispersedthroughout a solvent is referred to as electronic ink. This electronicink can be printed on a surface of glass, plastic, cloth, paper, or thelike. Furthermore, by use of a color filter or particles that have apigment, color display is possible, as well.

In addition, if a plurality of the aforementioned microcapsules arearranged as appropriate over an active matrix substrate so as to beinterposed between two electrodes, an active matrix display device canbe completed, and display can be performed by application of an electricfield to the microcapsules. For example, the active matrix substrateobtained with the thin film transistor described in Embodiment 3 and thedriver circuit described in Embodiment 2 can be used.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, or amagnetophoretic material or formed of a composite material of any ofthese.

Through the above steps, a highly reliable display device as asemiconductor device can be manufactured.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 6

A thin film transistor is manufactured using an oxide semiconductorlayer, and a semiconductor device having a display function (alsoreferred to as a “display device”) can be manufactured using the thinfilm transistor in a pixel portion and further in a driver circuit.Further, part or whole of a driver circuit can be formed over the samesubstrate as a pixel portion, using the inverter circuit described inEmbodiment 1 or 2, whereby a system-on-panel can be obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a “liquid crystal displayelement”) or a light-emitting element (also referred to as a“light-emitting display element”) can be used. Light-emitting elementsinclude, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically include an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by an electriceffect, such as an electronic ink, can be used.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC including a controller orthe like is mounted on the panel. An embodiment of the present inventionrelates to one embodiment of an element substrate before the displayelement is completed in a manufacturing process of the display device,and the element substrate is provided with means for supplying currentto the display element in each of a plurality of pixels. Specifically,the element substrate may be in a state provided with only a pixelelectrode of the display element, a state after a conductive film to bea pixel electrode is formed and before the conductive film is etched toform the pixel electrode, or any of other states.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device includes any of the followingmodules in its category: a module to which a connector such as aflexible printed circuit (FPC), tape automated bonding (TAB) tape, or atape carrier package (TCP) is attached; a module having TAB tape or aTCP which is provided with a printed wiring board at the end thereof;and a module having an integrated circuit (IC) which is directly mountedon a display element by a chip on glass (COG) method.

In this embodiment, the appearance and a cross section of a liquidcrystal display panel, which is an embodiment of a semiconductor deviceof the present invention, will be described with reference to FIGS. 19Ato 19C. FIGS. 19A and 19B are top views of a panel in which thin filmtransistors 4010 and 4011 with high electrical characteristics eachincluding an oxide semiconductor layer over a gate insulating layer, asource electrode layer, and a drain electrode layer, and a liquidcrystal element 4013 formed over a first substrate 4001 are sealedbetween the first substrate 4001 and a second substrate 4006 with asealant 4005. FIG. 19C is a cross-sectional view taken along the lineM-N of FIGS. 19A and 19B.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda gate line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the gate line driver circuit 4004. Therefore, the pixelportion 4002 and the gate line driver circuit 4004 are sealed togetherwith a liquid crystal layer 4008, by the first substrate 4001, thesealant 4005, and the second substrate 4006. A source line drivercircuit 4003 that is formed using a single crystal semiconductor film ora polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that the connection method of a driver circuit which is separatelyformed is not particularly limited, and a COG method, a wire bondingmethod, a TAB method, or the like can be used. FIG. 19A illustrates anexample of mounting the source line driver circuit 4003 by a COG method,and FIG. 19B illustrates an example of mounting the source line drivercircuit 4003 by a TAB method.

The pixel portion 4002 and the gate line driver circuit 4004 providedover the first substrate 4001 include a plurality of thin filmtransistors. FIG. 19C illustrates the thin film transistor 4010 includedin the pixel portion 4002 and the thin film transistor 4011 included inthe gate line driver circuit 4004. Over the thin film transistors 4010and 4011, insulating layers 4020 and 4021 are provided.

Each of the thin film transistors 4010 and 4011 corresponds to a thinfilm transistor with high electrical characteristics including an oxidesemiconductor layer over a gate insulating layer, a source electrodelayer, and a drain electrode layer, and the thin film transistor 170described in Embodiment 3 can be employed as the thin film transistors4010 and 4011. In this embodiment, the thin film transistors 4010 and4011 are n-channel thin film transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode layer 4030,the counter electrode layer 4031, and the liquid crystal layer 4008overlap one another corresponds to the liquid crystal element 4013. Notethat the pixel electrode layer 4030 and the counter electrode layer 4031are provided with an insulating layer 4032 and an insulating layer 4033respectively which each function as an alignment film, and the liquidcrystal layer 4008 is sandwiched between the pixel electrode layer 4030and the counter electrode layer 4031 with the insulating layers 4032 and4033 interposed therebetween.

Note that the first substrate 4001 and the second substrate 4006 can beformed using glass, metal (typically, stainless steel), ceramic, orplastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. In addition, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films can beused.

Reference numeral 4035 denotes a columnar spacer obtained by selectiveetching of an insulating film and is provided to control the distancebetween the pixel electrode layer 4030 and the counter electrode layer4031 (a cell gap). Further, a spherical spacer may also be used.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase is generated within an onlynarrow range of temperature, liquid crystal composition containing achiral agent at 5 wt % or more so as to improve the temperature range isused for the liquid crystal layer 4008. The liquid crystal compositionwhich includes liquid crystal exhibiting a blue phase and a chiral agenthave such characteristics that the response time is 10 μs to 100 μs,which is short, the alignment process is unnecessary because the liquidcrystal composition has optical isotropy, and viewing angle dependencyis small.

Although an example of a transmissive liquid crystal display device isdescribed in this embodiment, an embodiment of the present invention canalso be applied to a reflective liquid crystal display device and atransflective liquid crystal display device.

While an example of the liquid crystal display device in which thepolarizing plate is provided on the outer side of the substrate (on theviewer side) and the coloring layer and the electrode layer used for adisplay element are provided on the inner side of the substrate in thatorder is described in this embodiment, the polarizing plate may beprovided on the inner side of the substrate. The stacked structure ofthe polarizing plate and the coloring layer is not limited to thisembodiment and may be set as appropriate depending on materials of thepolarizing plate and the coloring layer or conditions of manufacturingsteps. Further, a light-blocking film serving as a black matrix may beprovided.

In this embodiment, in order to reduce surface unevenness of the thinfilm transistor and to improve reliability of the thin film transistor,the thin film transistor 170 obtained in Embodiment 3 is covered withthe insulating layers (the insulating layer 4020 and the insulatinglayer 4021) functioning as a protective film or a planarizing insulatingfilm. Note that the protective film is provided to prevent entry ofcontaminant impurities such as an organic substance, a metal, ormoisture floating in air and is preferably a dense film. The protectivefilm may be formed with a single layer or a stacked layer of a siliconoxide film, a silicon nitride film, a silicon oxynitride film, a siliconnitride oxide film, an aluminum oxide film, an aluminum nitride film,aluminum oxynitride film, and/or an aluminum nitride oxide film by asputtering method. Although an example in which the protective film isformed by a sputtering method is described in this embodiment, thepresent invention is not limited to this example, and the protectivefilm may be formed by a variety of methods.

In this embodiment, the insulating layer 4020 having a stacked-layerstructure is formed as a protective film. Here, as a first layer of theinsulating layer 4020, a silicon oxide film is formed by a sputteringmethod or a plasma CVD method. The use of a silicon oxide film as aprotective film has an effect of preventing hillock of an aluminum film.

As a second layer of the protective film, an insulating layer is formed.In this embodiment, as the second layer of the insulating layer 4020, asilicon nitride film is formed by a plasma CVD method. The use of thesilicon nitride film as the protective film can prevent mobile ions ofsodium or the like from entering a semiconductor region so thatvariation in electrical characteristics of the TFT can be suppressed.

After the protective film is formed, the oxide semiconductor layer maybe subjected to annealing (300° C. to 400° C.).

The insulating layer 4021 is formed as the planarizing insulating film.As the insulating layer 4021, an organic material having heat resistancesuch as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can beused. Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like. A siloxane-based resin may include, as a substituent, anorganic group (e.g., an alkyl group, and an aryl group) or a fluorogroup. The organic group may include a fluoro group. Note that theinsulating layer 4021 may be formed by stacking a plurality ofinsulating films formed of these materials.

Note that a siloxane-based resin is a resin formed from a siloxanematerial as a starting material and having the bond of Si—O—Si.

A formation method of the insulating layer 4021 is not particularlylimited, and the following method can be employed depending on thematerial: a sputtering method, an SOG method, a spin coating method, adipping method, a spray coating method, a droplet discharge method(e.g., an ink-jet method, screen printing, offset printing, or thelike), a doctor knife, a roll coater, a curtain coater, a knife coater,or the like. In the case of forming the insulating layer 4021 using amaterial solution, annealing (300° C. to 400° C.) of the oxidesemiconductor layer may be performed at the same time as a baking step.The baking step of the insulating layer 4021 also serves as annealing ofthe oxide semiconductor layer, whereby a semiconductor device can bemanufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide including tungsten oxide, indium zinc oxide including tungstenoxide, indium oxide including titanium oxide, indium tin oxide includingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, indium tin oxide to which silicon oxide is added, orthe like.

A conductive composition including a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed using the conductive composition preferably has a sheetresistance of less than or equal to 10000 ohms per square and atransmittance of greater than or equal to 70% at a wavelength of 550 nm.Further, the resistivity of the conductive high molecule included in theconductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of two or more kinds of them, and thelike can be given.

Further, a variety of signals and potentials are supplied to the sourceline driver circuit 4003 which is formed separately, the gate linedriver circuit 4004, or the pixel portion 4002 from an FPC 4018.

In this embodiment, a connection terminal electrode 4015 is formed fromthe same conductive film as that of the pixel electrode layer 4030included in the liquid crystal element 4013, and a terminal electrode4016 is formed from the same conductive film as that of the source anddrain electrode layers of the thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

FIGS. 19A to 19C illustrate an example in which the source line drivercircuit 4003 is formed separately and mounted on the first substrate4001; however, this embodiment is not limited to this structure. Thegate line driver circuit may be separately formed and then mounted, oronly part of the source line driver circuit or part of the gate linedriver circuit may be separately formed and then mounted.

FIG. 20 illustrates an example in which a liquid crystal display moduleis formed as a semiconductor device with use of a TFT substrate 2600manufactured according to an embodiment of the present invention.

FIG. 20 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTor the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the case of the RGB system, respective coloring layerscorresponding to colors of red, green, and blue are provided forrespective pixels. Polarizing plates 2606 and 2607 and a diffusion plate2613 are provided outside the TFT substrate 2600 and the countersubstrate 2601. A light source includes a cold cathode tube 2610 and areflective plate 2611, and a circuit substrate 2612 is connected to awiring circuit portion 2608 of the TFT substrate 2600 through a flexiblewiring board 2609 and includes an external circuit such as a controlcircuit or a power source circuit. The polarizing plate and the liquidcrystal layer may be stacked with a retardation plate interposedtherebetween.

For the liquid crystal display module, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, amulti-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, an axially symmetric aligned micro-cell (ASM)mode, an optical compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC)mode, or the like can be used.

Through the above steps, a highly reliable liquid crystal display deviceas a semiconductor device can be manufactured.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 7

In this embodiment, an example of a light-emitting display device willbe described as a semiconductor device of an embodiment of the presentinvention. As a display element included in a display device, alight-emitting element utilizing electroluminescence is described here.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade here using an organic EL element as a light-emitting element.

FIG. 21 illustrates an example of a pixel structure to which digitaltime grayscale driving can be applied, as an example of a semiconductordevice to which an embodiment of the present invention is applied.

A structure and operation of a pixel to which digital time grayscaledriving can be applied are described. Here, one pixel includes twon-channel transistors each of which includes an oxide semiconductorlayer as a channel formation region.

A pixel 6400 includes a switching transistor 6401, a driver transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driver transistor 6402. Note that a contact hole for directlyconnecting the second electrode to the gate of the driver transistor6402 can be formed by etching of a gate insulating layer, which isdescribed in Embodiment 2, whereby the total number of photomasks is notincreased. The gate of the driver transistor 6402 is connected to apower supply line 6407 through the capacitor 6403, a first electrode ofthe driver transistor 6402 is connected to the power supply line 6407,and a second electrode of the driver transistor 6402 is connected to afirst electrode (pixel electrode) of the light-emitting element 6404.

The second electrode (common electrode 6408) of the light-emittingelement 6404 is set to a low power supply potential. Note that the lowpower supply potential is a potential satisfying the low power supplypotential<a high power supply potential with reference to the high powersupply potential that is set to the power supply line 6407. As the lowpower supply potential, GND, 0 V, or the like may be employed, forexample. A potential difference between the high power supply potentialand the low power supply potential is applied to the light-emittingelement 6404 and current is supplied to the light-emitting element 6404,so that the light-emitting element 6404 emits light. Here, in order tomake the light-emitting element 6404 emit light, each potential is setso that the potential difference between the high power supply potentialand the low power supply potential is a forward threshold voltage orhigher.

Note that gate capacitance of the driver transistor 6402 may be used asa substitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitance of the driver transistor 6402 may beformed between the channel region and the gate electrode.

In the case of a voltage-input voltage driving method, a video signal isinput to the gate of the driver transistor 6402 so that the drivertransistor 6402 is in either of two states of being sufficiently turnedon and turned off. That is, the driver transistor 6402 operates in alinear region. Since the driver transistor 6402 operates in a linearregion, a voltage higher than the voltage of the power supply line 6407is applied to the gate of the driver transistor 6402. Note that avoltage higher than or equal to (voltage of the power supply line+Vth ofthe driver transistor 6402) is applied to the signal line 6405.

In the case of performing analog grayscale driving instead of digitaltime grayscale driving, the same pixel structure as that in FIG. 21 canbe used by changing signal input.

In the case of performing analog grayscale driving, a voltage higherthan or equal to (forward voltage of the light-emitting element 6404+Vthof the driver transistor 6402) is applied to the gate of the drivertransistor 6402. The forward voltage of the light-emitting element 6404indicates a voltage at which a desired luminance is obtained, andincludes at least forward threshold voltage. The video signal by whichthe driver transistor 6402 operates in a saturation region is input, sothat current can be supplied to the light-emitting element 6404. Inorder for the driver transistor 6402 to operate in a saturation region,the potential of the power supply line 6407 is set higher than the gatepotential of the driver transistor 6402. When an analog video signal isused, it is possible to feed current to the light-emitting element 6404in accordance with the video signal and perform analog grayscaledriving.

Note that the pixel structure illustrated in FIG. 21 is not limitedthereto. For example, a switch, a resistor, a capacitor, a transistor, alogic circuit, or the like may be added to the pixel illustrated in FIG.21.

Next, structures of the light-emitting element will be described withreference to FIGS. 22A to 22C. A cross-sectional structure of a pixelwill be described by taking an enhancement type driving TFT as anexample. Driving TFTs 7001, 7011, and 7021 used for semiconductordevices illustrated in FIGS. 22A to 22C can be manufactured in a mannersimilar to the thin film transistor described in Embodiment 3 and arehighly reliable thin film transistors each including an oxidesemiconductor layer over a gate insulating layer, a source electrodelayer, and a drain electrode layer.

In order to extract light emitted from the light-emitting element, atleast one of the anode and the cathode is required to transmit light. Athin film transistor and a light-emitting element are formed over asubstrate. A light-emitting element can have a top emission structure,in which light emission is extracted through the surface opposite to thesubstrate; a bottom emission structure, in which light emission isextracted through the surface on the substrate side; or a dual emissionstructure, in which light emission is extracted through the surfaceopposite to the substrate and the surface on the substrate side. A pixelstructure can be applied to a light-emitting element having any of theseemission structures.

A light-emitting element having a top emission structure will bedescribed with reference to FIG. 22A.

FIG. 22A is a cross-sectional view of a pixel in a case where thedriving TFT 7001 is an n-channel TFT and light is emitted from alight-emitting element 7002 to an anode 7005 side. In FIG. 22A, acathode 7003 of the light-emitting element 7002 is electricallyconnected to the driving TFT 7001, and a light-emitting layer 7004 andthe anode 7005 are stacked in this order over the cathode 7003. Thecathode 7003 can be formed using a variety of conductive materials aslong as they have a low work function and reflect light. For example,Ca, Al, CaF, MgAg, AlLi, or the like is preferably used. Thelight-emitting layer 7004 may be formed using a single layer or aplurality of layers stacked. When the light-emitting layer 7004 isformed using a plurality of layers, the light-emitting layer 7004 isformed by stacking an electron-injecting layer, an electron-transportinglayer, a light-emitting layer, a hole-transporting layer, and ahole-injecting layer in this order over the cathode 7003. It is notnecessary to form all of these layers. The anode 7005 is formed using alight-transmitting conductive film such as a film of indium oxideincluding tungsten oxide, indium zinc oxide including tungsten oxide,indium oxide including titanium oxide, indium tin oxide includingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

The light-emitting element 7002 corresponds to a region where thelight-emitting layer 7004 is sandwiched between the cathode 7003 and theanode 7005. In the case of the pixel illustrated in FIG. 22A, light isemitted from the light-emitting element 7002 to the anode 7005 side asindicated by an arrow.

Next, a light-emitting element having a bottom emission structure willbe described with reference to FIG. 22B. FIG. 22B is a cross-sectionalview of a pixel in a case where the driving TFT 7011 is an n-channeltransistor and light is emitted from a light-emitting element 7012 to acathode 7013 side. In FIG. 22B, the cathode 7013 of the light-emittingelement 7012 is formed over a light-transmitting conductive film 7017that is electrically connected to the driving TFT 7011, and alight-emitting layer 7014 and an anode 7015 are stacked in this orderover the cathode 7013. A light-blocking film 7016 for reflecting orblocking light may be formed to cover the anode 7015 when the anode 7015has a light-transmitting property. For the cathode 7013, a variety ofmaterials can be used as in the case of FIG. 22A as long as they areconductive materials having a low work function. The cathode 7013 isformed to have a thickness that can transmit light (preferably,approximately 5 nm to 30 nm). For example, an aluminum film with athickness of 20 nm can be used as the cathode 7013. Similar to the caseof FIG. 19A, the light-emitting layer 7014 may be formed using either asingle layer or a plurality of layers stacked. The anode 7015 is notrequired to transmit light, but can be formed using a light-transmittingconductive material as in the case of FIG. 22A. As the light-blockingfilm 7016, a metal or the like that reflects light can be used forexample; however, it is not limited to a metal film. For example, aresin or the like to which black pigments are added can also be used.

The light-emitting element 7012 corresponds to a region where thelight-emitting layer 7014 is sandwiched between the cathode 7013 and theanode 7015. In the case of the pixel illustrated in FIG. 22B, light isemitted from the light-emitting element 7012 to the cathode 7013 side asindicated by an arrow.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 22C. In FIG. 22C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. As in the case of FIG. 22A, thecathode 7023 can be formed using a variety of conductive materials aslong as they have a low work function. The cathode 7023 is formed tohave a thickness that can transmit light. For example, a film of Alhaving a thickness of 20 nm can be used as the cathode 7023. As in FIG.22A, the light-emitting layer 7024 may be formed using either a singlelayer or a plurality of layers stacked. The anode 7025 can be formedusing a light-transmitting conductive material as in the case of FIG.22A.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 overlapwith one another. In the case of the pixel illustrated in FIG. 22C,light is emitted from the light-emitting element 7022 to both the anode7025 side and the cathode 7023 side as indicated by arrows.

Note that, although an organic EL element is described here as alight-emitting element, an inorganic EL element can also be provided asa light-emitting element.

In this embodiment, the example is described in which a thin filmtransistor (a driving TFT) which controls the driving of alight-emitting element is electrically connected to the light-emittingelement; however, a structure may be employed in which a TFT for currentcontrol is connected between the driving TFT and the light-emittingelement.

A semiconductor device described in this embodiment is not limited tothe structures illustrated in FIGS. 22A to 22C and can be modified invarious ways based on the spirit of techniques according to the presentinvention.

Next, the appearance and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel), which is oneembodiment of a semiconductor device of the present invention, will bedescribed with reference to FIGS. 23A and 23B. FIG. 23A is a top view ofa panel in which a thin film transistor with high electricalcharacteristics including a gate insulating layer over a firstsubstrate, a source and drain electrode layers over the gate insulatinglayer, and an oxide semiconductor layer over the source and drainelectrode layers and a light-emitting element are sealed between thefirst substrate and a second substrate with a sealant. FIG. 23B is across-sectional view taken along the line H-I of FIG. 22A.

A sealant 4505 is provided so as to surround a pixel portion 4502,source line driver circuits 4503 a and 4503 b, and gate line drivercircuits 4504 a and 4504 b which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the source line driver circuits 4503 a and 4503 b, and thegate line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the source line driver circuits 4503 a and 4503 b, and thegate line driver circuits 4504 a and 4504 b are sealed together with afiller 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. It is preferable that a panel be packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air as described above.

The pixel portion 4502, the source line driver circuits 4503 a and 4503b, and the gate line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors,and a thin film transistor 4510 included in the pixel portion 4502 and athin film transistor 4509 included in the source line driver circuit4503 a are illustrated as an example in FIG. 23B.

Each of the thin film transistors 4509 and 4510 corresponds to a highlyreliable thin film transistor including an oxygen-excess oxidesemiconductor layer over a gate insulating layer, a source electrodelayer, a drain electrode layer, a source region, and a drain regionwhich have been subjected to oxygen radical treatment, and includingoxygen-deficient oxide semiconductor layers as the source region and thedrain region, and the thin film transistor described in Embodiments 3can be employed as the thin film transistors 4509 and 4510. In thisembodiment, the thin film transistors 4509 and 4510 are n-channel thinfilm transistors.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that a structure of the light-emitting element 4511 is astacked-layer structure of the first electrode layer 4517, theelectroluminescent layer 4512, and the second electrode layer 4513, butthe present invention is not limited to that described in thisembodiment. The structure of the light-emitting element 4511 can bechanged as appropriate depending on the direction in which light isextracted from the light-emitting element 4511, or the like.

A partition wall 4520 is formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. It is particularlypreferable that the partition wall 4520 be formed using a photosensitivematerial and an opening be formed over the first electrode layer 4517 sothat a sidewall of the opening is formed as an inclined surface withcontinuous curvature.

The electroluminescent layer 4512 may be formed with a single layer or aplurality of layers stacked.

A protective film may be formed over the second electrode layer 4513 andthe partition wall 4520 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4511. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

In addition, a variety of signals and potentials are supplied to thesource line driver circuits 4503 a and 4503 b, the gate line drivercircuits 4504 a and 4504 b, or the pixel portion 4502 from FPCs 4518 aand 4518 b.

In Embodiment 7, a connection terminal electrode 4515 is formed from thesame conductive film as the first electrode layer 4517 included in thelight-emitting element 4511, and a terminal electrode 4516 is formedfrom the same conductive film as the source and drain electrode layersincluded in the thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

The second substrate 4506 located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, PVC (polyvinyl chloride), acrylic, polyimide, anepoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylenevinyl acetate) can be used. In Embodiment 7, nitrogen is used for thefiller 4507.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

The source line driver circuits 4503 a and 4503 b and the gate linedriver circuits 4504 a and 4504 b may be provided as driver circuitsformed using a single crystal semiconductor film or polycrystallinesemiconductor film over a substrate separately prepared. In addition,only the source line driver circuits or part thereof, or the gate linedriver circuits or part thereof may be separately formed and mounted.This embodiment is not limited to the structure illustrated in FIGS. 23Aand 23B.

Through the above steps, a highly reliable light-emitting display device(display panel) as a semiconductor device can be manufactured.

Embodiment 7 can be combined with any of the other embodiments asappropriate.

Embodiment 8

A semiconductor device of an embodiment of the present invention can beapplied to an electronic paper. An electronic paper can be used forelectronic devices of a variety of fields as long as they can displaydata. For example, an electronic paper can be applied to an electronicbook (e-book) reader, a poster, an advertisement in a vehicle such as atrain, displays of various cards such as a credit card, and the like.Examples of the electronic devices are illustrated in FIGS. 24A and 24Band FIG. 25.

FIG. 24A illustrates a poster 2631 formed using an electronic paper. Ina case where an advertising medium is printed paper, the advertisementis replaced by manpower; however, by use of an electronic paper to whichthe present invention is applied, the advertising display can be changedin a short time. Further, an image can be stably displayed without beingdistorted. Note that the poster may be configured to transmit andreceive data wirelessly.

FIG. 24B illustrates an advertisement 2632 in a vehicle such as a train.In a case where an advertising medium is printed paper, theadvertisement is replaced by manpower; however, by use of an electronicpaper to which an embodiment of the present invention is applied, theadvertising display can be changed in a short time without a lot ofmanpower. Further, an image can be stably displayed without beingdistorted. Note that the advertisement in a vehicle may be configured totransmit and receive data wirelessly.

FIG. 25 illustrates an example of an electronic book reader 2700. Forexample, the electronic book reader 2700 includes two housings, ahousing 2701 and a housing 2703. The housing 2701 and the housing 2703are combined with a hinge 2711 so that the electronic book reader 2700can be opened and closed with the hinge 2711 as an axis. With such astructure, the electronic book reader 2700 can be operated like a paperbook.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may be configured to display one imageor different images. In a case where the display portion 2705 and thedisplay portion 2707 display different images, for example, a displayportion on the right side (the display portion 2705 in FIG. 25) candisplay text and a display portion on the left side (the display portion2707 in FIG. 25) can display graphics.

FIG. 25 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, or the like may be provided onthe surface of the housing, on which the display portion is provided.Further, an external connection terminal (an earphone terminal, a USBterminal, a terminal that can be connected to various cables such as anAC adapter and a USB cable, or the like), a recording medium insertportion, or the like may be provided on the back surface or the sidesurface of the housing. Further, the electronic book reader 2700 mayhave a function of an electronic dictionary.

The electronic book reader 2700 may be configured to transmit andreceive data wirelessly. The structure can be employed in which desiredbook data or the like is purchased and downloaded from an electronicbook server wirelessly.

Embodiment 8 can be combined with any of the other embodiments asappropriate.

Embodiment 9

A semiconductor device according to an embodiment of the presentinvention can be applied to a variety of electronic devices (includingan amusement machine). Examples of electronic devices include atelevision set (also referred to as a “television” or a “televisionreceiver”), a monitor of a computer or the like, a camera such as adigital camera or a digital video camera, a digital photo frame, amobile phone handset (also referred to as a “mobile phone” or a “mobilephone device”), a portable game console, a portable informationterminal, an audio reproducing device, a large-sized game machine suchas a pachinko machine, and the like.

FIG. 26A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. The display portion 9603 can display an image. Further, thehousing 9601 is supported by a stand 9605 here.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with an operation key 9609 of the remote controller9610 so that an image displayed on the display portion 9603 can becontrolled. Further, the remote controller 9610 may be provided with adisplay portion 9607 for displaying data outputted from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the receiver, a general television broadcast can bereceived. Further, when the television set 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver or between receivers) data communication canbe performed.

FIG. 26B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. The display portion 9703 can displayvarious images. For example, the display portion 9703 can display dataof an image taken with a digital camera or the like and function as anormal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection portion (a USB terminal, a terminal thatcan be connected to various cables such as a USB cable, or the like), arecording medium insertion portion, and the like. Although thesecomponents may be provided on the surface on which the display portionis provided, it is preferable to provide them on the side surface or theback surface for the design of the digital photo frame 9700. Forexample, a memory storing data of an image taken with a digital camerais inserted in the recording medium insertion portion of the digitalphoto frame, whereby the image data can be transferred and thendisplayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. The structure may be employed in which desired imagedata is transferred wirelessly to be displayed.

FIG. 27A is a portable game machine and includes two housings, a housing9881 and a housing 9891, which are connected with a joint portion 9893so that the portable game machine can be opened or folded. A displayportion 9882 is incorporated in the housing 9881, and a display portion9883 is incorporated in the housing 9891. In addition, the portable gamemachine illustrated in FIG. 27A is provided with a speaker portion 9884,a recording medium insert portion 9886, an LED lamp 9890, input means(operation keys 9885, a connection terminal 9887, a sensor 9888 (havinga function of measuring force, displacement, position, speed,acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, odor, or infrared ray), and amicrophone 9889), and the like. Needless to say, the structure of theportable game machine is not limited to that described above. Theportable game machine may have a structure in which additional accessoryequipment is provided as appropriate as long as at least a semiconductordevice according to an embodiment of the present invention is provided.The portable game machine illustrated in FIG. 27A has a function ofreading a program or data stored in a recording medium to display it onthe display portion, and a function of sharing information with anotherportable game machine by wireless communication. Note that a function ofthe portable game machine illustrated in FIG. 27A is not limited tothose described above, and the portable game machine can have a varietyof functions.

FIG. 27B illustrates an example of a slot machine 9900 which is alarge-sized amusement machine. In the slot machine 9900, a displayportion 9903 is incorporated in a housing 9901. In addition, the slotmachine 9900 is provided with operation means such as a start lever anda stop switch, a coin slot, a speaker, or the like. Needless to say, thestructure of the slot machine 9900 is not limited to the above-describedstructure. The slot machine may have a structure in which additionalaccessory equipment is provided as appropriate as long as at least asemiconductor device according to the present invention is provided.

FIG. 28A illustrates an example of a mobile phone handset 1000. Themobile phone handset 1000 is provided with a display portion 1002incorporated in a housing 1001, operation buttons 1003, an externalconnection port 1004, a speaker 1005, a microphone 1006, and the like.

When the display portion 1002 of the mobile phone handset 1000illustrated in FIG. 28A is touched with a finger or the like, data canbe input into the mobile phone handset 1000. Further, operations such asmaking calls and texting can be performed by touching the displayportion 1002 with a finger or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode which is a combination of the twomodes, that is, a combination of the display mode and the input mode.

For example, in the case of making a call or texting, a text input modemainly for inputting text is selected for the display portion 1002 sothat characters displayed on a screen can be inputted. In that case, itis preferable to display a keyboard or number buttons on almost all areaof the screen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone handset 1000, display on the screen of the display portion1002 can be automatically changed by determining the orientation of themobile phone handset 1000 (whether the mobile phone handset 1000 isplaced horizontally or vertically for a landscape mode or a portraitmode).

The screen modes are changed by touching the display portion 1002 orusing the operation buttons 1003 of the housing 1001. Alternatively, thescreen modes may be changed depending on the kind of the image displayedon the display portion 1002. For example, when a signal of an imagedisplayed on the display portion is the one of moving image data, thescreen mode is changed to the display mode. When the signal is the oneof text data, the screen mode is changed to the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 1002 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken when thedisplay portion 1002 is touched with a palm or a finger, wherebypersonal identification can be performed. Further, when a backlight or asensing light source which emits a near-infrared light in the displayportion is used, an image of a finger vein, a palm vein, or the like canbe taken.

FIG. 28B also illustrates an example of a mobile phone. The mobile phoneillustrated in FIG. 28B has a display device 9410 having a displayportion 9412 and operation buttons 9413 in a housing 9411 and acommunication device 9400 having operation buttons 9402, an externalinput terminal 9403, a microphone 9404, a speaker 9405, and alight-emitting portion 9406 which emits light when receiving a call in ahousing 9401. The display device 9410 having a display function can bedetached from or attached to the communication device 9400 having atelephone function in two directions indicated by the arrows.Accordingly, the display device 9410 and the communication device 9400can be attached to each other along their short sides or long sides. Inaddition, in a case where only the display function is needed, thedisplay device 9410 is detached from the communication device 9400 sothat the display device 9410 can be used by itself. The communicationdevice 9400 and the display device 9410 are capable of sending andreceiving images or input information by a wireless communication orwire communication. The communication device 9400 and the display device9410 each have a rechargeable battery.

Embodiment 9 can be combined with any of the other embodiments asappropriate.

Embodiment 10

In Embodiment 10, an example in which a 4-inch QVGA liquid crystaldisplay panel is actually manufactured will be described.

A bottom-gate bottom-contact TFT that can be obtained through theprocess described in Embodiment 3 is advantageous for an increase inproductivity and high-speed driving of a source line driver circuit, inwhich case a source electrode wiring and a drain electrode wiring over agate insulating film can be patterned so as to have a size designed byphotolithography and dry etching, which enables control of a channellength and miniaturization. In addition, as illustrated in FIG. 5C, astorage capacitor C_(s) which is placed in a pixel of a liquid crystaldisplay device can be formed of a capacitor wiring, a gate insulatinglayer, a protective film, and a pixel electrode without using an oxidesemiconductor layer (an In—Ga—Zn—O non-single-crystal film), andtherefore large capacitance can be secured in a small area. In the caseof a 4-inch panel having a display standard of QVGA, opening aperturecan be increased by 4%. Moreover, using a structure in which the sourceor drain electrode is directly connected to the gate electrode through acontact hole formed in the gate insulating film (such a structure isalso referred to as a “direct contact structure”) makes it possible toreduce the number of contacts in a shift register. The reduction in thenumber of contacts enables an increase in yield.

FIG. 29 shows measurement results of TFTs (32 randomly selected TFTsover the same substrate) that are actually manufactured in accordancewith the process described in Embodiment 3. Conditions of each of theTFTs are as follows: the thickness of a gate oxide film (relativedielectric constant: 4.1) was 200 nm, the channel length L was 4 μm, thechannel width W was 20 μm, and each of the 32 TFTs over the samesubstrate was measured. In FIG. 29, V_(G)-I_(D) curves of the 32 TFTsover the same substrate almost overlap, which shows that TFTs thatexhibit little variation can be obtained. Field effect mobility μFE wascalculated from the VG-ID curves in FIG. 29. The field effect mobilityis calculated on the assumption of gradual channel approximation, and ina saturation region (Vds=10 V), the value of the TFT out of 32 TFTswhich shows the maximum field effect mobility (μFE) is 11.3 cm²/Vs.

A shift register consisting of a plurality of units illustrated in FIG.17 and FIG. 18 is used for a driver circuit. The driver circuit isdesigned so as to be driven at a voltage of 16 V and needs two positivepower supplies and one negative power supply. The channel length L andthe channel width W of a TFT of the driver circuit are 10 μm and 50 μm,respectively. The number of stages of the shift registers in the drivercircuit was 44. FIG. 30 shows results of measurement of the actuallymanufactured driver circuit with an oscilloscope. In FIG. 30, the topwaveform is an output waveform of the last stage (Unit 44) of the shiftregister, the waveform under Unit 44 is an output waveform of is Unit43, and the waveform under Unit 43 is an output waveform of Unit 42. Thedriving voltage is 16 V. Consumption current at this time is 0.57 mA.The bottom waveform in FIG. 30 is one of the waveforms in the four-phaseclock, and part of the waveform is outputted from Unit 42 of the shiftregister. In a case where the shift register is used for a gate linedriver circuit, the following are required: a driver driving frequencyof 3.66 kHz and a gate selection period of less than or equal to 68.31μs in the case of a panel of the display standard of QVGA, and a gateselection period of less than or equal to 34.44 μs in the case of apanel of the display standard of VGA. It is found that the drivercircuit of Embodiment 10 satisfies the above specifications.

Next, FIG. 31 shows driver output waveforms at the maximum drivingfrequency (606.2 kHz). The fourth stage waveform is one of the waveformsin the four-phase clock, and part of the waveform is outputted from Unit42 of the driver circuit. In the case of a panel of the display standardof QVGA, the driver driving frequency is 234.24 kHz, and when a videosignal is written with the driver output waveform using this drivercircuit, the writing period is 1.07 μs. According to this result, anincrease in the number of video signals and transmission of video datato a panel by division input enable a pixel portion, a gate line drivercircuit, and a source line driver circuit to be mounted on the samesubstrate. In Embodiment 10, the number of video signals is 16.

A driver circuit including the above shift registers and a pixel portionwere formed over the same substrate to manufacture a 4-inch full-colorliquid crystal display. The specifications of the display are shown inTable 3.

TABLE 3 Items Specification Diagonal size 4.015 inches No. of pixels 320× RGB × 240 (QVGA) Resolution 99.6 dpi Panel size 8.74 cm × 9.94 cmDriver Integration Aperture ratio 41.8%

The number of pixels of the manufactured display is 320×RGB×240 (QVGA).The pixel density thereof is 99.6 dpi. The manufactured display is adisplay incorporating the source line driver circuit and the gate linedriver circuit.

The clock frequency of the gate line driver circuit was 3.66 kHz and thegate selection period thereof was 68.31 μs. Sixteen video signals areanalog-inputted at the same time and written to the panel through aswitch. The video writing period was 1.07 [μs] and the driving frequencyof the source line driver circuit was 234.24 kHz. A 4.015-inchfull-color active matrix liquid crystal display was manufacturedexperimentally. FIG. 32 illustrates the display. Since a display portionand the driver circuits are formed over the same substrate, a sourceline driver circuit 201 and a gate line driver circuit 202 are includedin the periphery of a display region as illustrated in FIG. 32.

This application is based on Japanese Patent Application serial no.2008-259064 filed with Japan Patent Office on Oct. 3, 2008 and JapanesePatent Application serial no. 2009-150998 filed with Japan Patent Officeon Jun. 25, 2009, the entire contents of which are hereby incorporatedby reference.

1. A display device comprising: a pixel portion comprising a first thinfilm transistor comprising a first oxide semiconductor layer; a drivercircuit comprising a second thin film transistor comprising a secondoxide semiconductor layer and a third thin film transistor comprising athird oxide semiconductor layer, the second thin film transistorcomprising a gate electrode below the second oxide semiconductor layer;and a terminal portion comprising a transparent conductive film, aterminal and a floating electrode, wherein the transparent conductivefilm is electrically connected to a source wiring through the terminal,wherein the floating electrode comprises a same material as the gateelectrode, wherein the terminal and the floating electrode overlap witheach other, wherein a wiring is directly connected to the gate electrodeof the second thin film transistor, and wherein the third oxidesemiconductor layer is directly connected to the wiring.
 2. The displaydevice according to claim 1, wherein the wiring is electricallyconnected to the gate electrode of the second thin film transistorthrough a contact hole formed in a gate insulating layer.
 3. The displaydevice according to claim 1, wherein the first oxide semiconductorlayer, the second oxide semiconductor layer, and the third oxidesemiconductor layer contain indium, gallium, and zinc.
 4. The displaydevice according to claim 1, wherein the pixel portion and the drivercircuit are provided over the same substrate.
 5. A display devicecomprising: a pixel portion over a substrate, the pixel portioncomprising: a first thin film transistor comprising a first gateelectrode and a first oxide semiconductor layer over the first gateelectrode with a gate insulating layer between the first gate electrodeand the first oxide semiconductor layer; a driver circuit over thesubstrate, the driver circuit comprising: a wiring over the gateinsulating layer; and a second thin film transistor comprising a secondgate electrode and a second oxide semiconductor layer over the secondgate electrode with the gate insulating layer between the second gateelectrode and the second oxide semiconductor layer, wherein the secondoxide semiconductor layer overlaps the wiring; and a terminal portioncomprising a transparent conductive film, a terminal and a floatingelectrode, wherein the transparent conductive film is electricallyconnected to a source wiring through the terminal, wherein the floatingelectrode comprises a same material as the gate electrode, wherein theterminal and the floating electrode overlap with each other, and whereinthe wiring is in contact with the second oxide semiconductor layer andthe second gate electrode.
 6. The display device according to claim 5,wherein the wiring is in contact with the second gate electrode througha contact hole formed in the gate insulating layer.
 7. The displaydevice according to claim 5, wherein the first oxide semiconductor layerand the second oxide semiconductor layer contain indium, gallium, andzinc.
 8. A display device comprising: a pixel portion over a substrate,the pixel portion comprising: a first thin film transistor comprising afirst gate electrode and a first oxide semiconductor layer over thefirst gate electrode with a gate insulating layer between the first gateelectrode and the first oxide semiconductor layer; a driver circuit overthe substrate, the driver circuit comprising: a wiring over the gateinsulating layer; a second thin film transistor comprising a second gateelectrode and a second oxide semiconductor layer over the second gateelectrode with the gate insulating layer between the second gateelectrode and the second oxide semiconductor layer, wherein the secondoxide semiconductor layer overlaps the wiring; and a third thin filmtransistor comprising a third gate electrode and a third oxidesemiconductor layer over the third gate electrode with the gateinsulating layer between the third gate electrode and the third oxidesemiconductor layer, wherein the third oxide semiconductor layeroverlaps the wiring; and a terminal portion comprising a transparentconductive film, a terminal and a floating electrode, wherein thetransparent conductive film is electrically connected to a source wiringthrough the terminal, wherein the floating electrode comprises a samematerial as the gate electrode, wherein the terminal and the floatingelectrode overlap with each other, and wherein the wiring is in contactwith the second oxide semiconductor layer, the third oxide semiconductorlayer, and the third gate electrode.
 9. The display device according toclaim 8, wherein the wiring is in contact with the third gate electrodethrough a contact hole formed in the gate insulating layer.
 10. Thedisplay device according to claim 8, wherein the first oxidesemiconductor layer, the second oxide semiconductor layer, and the thirdoxide semiconductor layer contain indium, gallium, and zinc.